Add period to period_archives context. Refs #1044.

* Adds period tuple of (year, month, day) matching the time
  period of the current archive. Note that this is done
  to the archive context if period_archives.html doesn't exist.
* Adds tests to verify this.
* Adds documentation in themes.rst about period in period_archives.html.

pelican/themes/notmyidea/templates/period_archives.html

@@ -0,0 +1,13 @@
+{% extends "base.html" %}
+{% block content %}
+<section id="content" class="body">
+<h1>Archives for {{ period | reverse | join(' ') }}</h1>
+
+<dl>
+{% for article in dates %}
+    <dt>{{ article.locale_date }}</dt>
+    <dd><a href="{{ SITEURL }}/{{ article.url }}">{{ article.title }}</a></dd>
+{% endfor %}
+</dl>
+</section>
+{% endblock %}

pelican/themes/simple/templates/period_archives.html

@@ -0,0 +1,11 @@
+{% extends "base.html" %}
+{% block content %}
+<h1>Archives for {{ period | reverse | join(' ') }}</h1>
+
+<dl>
+{% for article in dates %}
+    <dt>{{ article.locale_date }}</dt>
+    <dd><a href="{{ SITEURL }}/{{ article.url }}">{{ article.title }}</a></dd>
+{% endfor %}
+</dl>
+{% endblock %}