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.

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