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.

I completed @dbrgn's work (Issue #1044.) and changed the *year*, *year_name*... variables to a *period* variable to be less verbose in the templates.