Add filter to calendar so posts can be filtered by given taxonomy.

First go at adding some filtering by taxonomy to the calendar in response to [this post](http://wordpress.org/support/topic/plugin-edit-flow-sort-calendar-story-budget-by-taxonomy) on the forums, and issue #105. 

Props to @samjco for kicking it off. Still need to do some more bug testing and fix style issues, but figured I'd toss it out here for comment.