* Add a calendar plugin, contributed by Manoj Srivastava.

* Add a calendar plugin, contributed by Manoj Srivastava.
* Reformat calendar plugin to ikiwiki conventions.
* The calendar plugin made *every* page depend on every other page,
  which seemed a wee tiny little bit overkill. Fixed the dependency
  calculations (I hope.)
* Removed manual ctime statting code, and just have the calendar plugin use
  %pagectime.