Add the possibility to add a subject line in the letter

This is the suggestion given by Matthias Baur on launchpad
https://answers.launchpad.net/moderncv/+question/222784 in comment 12
Closes #113

I included small layout tweaks in the different moderncvhead variants.

moderncv.cls

@@ -716,6 +716,7 @@
 % elements
 \newcommand*{\recipient}[2]{\def\@recipientname{#1}\def\@recipientaddress{#2}}
 \renewcommand*{\date}[1]{\def\@date{#1}}\date{\today}
+\newcommand*{\subject}[1]{\def\@subject{#1}}
 \newcommand*{\opening}[1]{\def\@opening{#1}}
 \newcommand*{\closing}[1]{\def\@closing{#1}}
 \newcommand*{\enclosure}[2][]{%

moderncvheadi.sty

@@ -161,7 +161,7 @@
         \ifthenelse{\isundefined{\@email}}{}{\makenewline\emailsymbol\emaillink{\@email}}%
         \ifthenelse{\isundefined{\@homepage}}{}{\makenewline\homepagesymbol\httpslink{\@homepage}}%
         \ifthenelse{\isundefined{\@extrainfo}}{}{\makenewline\@extrainfo}}\fi%
-    \end{minipage}\\[1em]
+    \end{minipage}\\[2em]
   % recipient block
   \begin{minipage}[t]{.5\textwidth}
     \raggedright%
@@ -173,6 +173,8 @@
   \hfill% US style
 %  \\[1em]% UK style
   \@date\\[2em]% US informal style: "January 1, 1900"; UK formal style: "01/01/1900"
+  % optional subject
+  \ifthenelse{\isundefined{\@subject}}{}{{\bfseries\@subject\\[2em]}}
   % opening
   \raggedright%
   \@opening\\[1.5em]%

moderncvheadii.sty

@@ -189,7 +189,9 @@
   % date
   \hfill% US style
 %  \\[1em]% UK style
-  \@date\\[2em]% US informal style: "January 1, 1900"; UK formal style: "01/01/1900"
+  \@date\\[3em]% US informal style: "January 1, 1900"; UK formal style: "01/01/1900"
+  % optional subject
+  \ifthenelse{\isundefined{\@subject}}{}{{\bfseries\@subject\\[3em]}}
   % opening
   \raggedright%
   \@opening\\[1.5em]%

moderncvheadiii.sty

@@ -145,7 +145,9 @@
   % date
   \hfill% US style
 %  \\[1em]% UK style
-  \@date\\[2em]% US informal style: "January 1, 1900"; UK formal style: "01/01/1900"
+  \@date\\[3em]% US informal style: "January 1, 1900"; UK formal style: "01/01/1900"
+  % optional subject
+  \ifthenelse{\isundefined{\@subject}}{}{{\bfseries\@subject\\[3em]}}
   % opening
   \raggedright%
   \@opening\\[1.5em]%

moderncvheadiv.sty

@@ -143,6 +143,8 @@
     \@recipientaddress}\\[1em]%
   % date
   \@date\\[2em]%
+  % optional subject
+  \ifthenelse{\isundefined{\@subject}}{}{{\bfseries\@subject\\[2em]}}
   % opening
   \@opening\\[1.5em]%
   % sender contact info

moderncvheadv.sty

@@ -148,7 +148,7 @@
         \ifthenelse{\isundefined{\@email}}{}{\makenewline\emailsymbol\emaillink{\@email}}%
         \ifthenelse{\isundefined{\@homepage}}{}{\makenewline\homepagesymbol\httpslink{\@homepage}}%
         \ifthenelse{\isundefined{\@extrainfo}}{}{\makenewline\@extrainfo}}\fi%
-    \end{minipage}\\[1em]
+    \end{minipage}\\[2em]
   % recipient block
   \begin{minipage}[t]{.5\textwidth}
     \raggedright%
@@ -159,7 +159,9 @@
   % date
   \hfill% US style
 %  \\[1em]% UK style
-  \@date\\[2em]% US informal style: "January 1, 1900"; UK formal style: "01/01/1900"
+  \@date\\[3em]% US informal style: "January 1, 1900"; UK formal style: "01/01/1900"
+  % optional subject
+  \ifthenelse{\isundefined{\@subject}}{}{{\bfseries\@subject\\[2em]}}
   % opening
   \raggedright%
   \@opening\\[1.5em]%

moderncvheadvi.sty

@@ -154,7 +154,9 @@
   % date
   \hfill% US style
 %  \\[1em]% UK style
-  \@date\\[2em]% US informal style: "January 1, 1900"; UK formal style: "01/01/1900"
+  \@date\\[3em]% US informal style: "January 1, 1900"; UK formal style: "01/01/1900"
+  % optional subject
+  \ifthenelse{\isundefined{\@subject}}{}{{\bfseries\@subject\\[3em]}}
   % opening
   \raggedright%
   \@opening\\[1.5em]%

template.tex

@@ -239,6 +239,7 @@ Detailed achievements:
 % recipient data
 \recipient{Company Recruitment team}{Company, Inc.\\123 somestreet\\some city}
 \date{January 01, 1984}
+\subject{Job application}
 \opening{Dear Sir or Madam,}
 \closing{Yours faithfully,}
 \enclosure[Attached]{curriculum vit\ae{}}          % use an optional argument to use a string other than "Enclosure", or redefine \enclname