Adding the iCal feed into the filter by terms functionality.

#2672 

This PR exporting events by the terms they tagged to. For example 'john/calendar/export.ics' will export all of the events but 'john/calendar/export.ics/authors/antoine-de-saint-exupéry' will export all of the events that they tagged to Antoine de Saint-Exupéry.

Another thing that this PR handle with is the term filtering in the calendar display. 
![selection_084](https://f.cloud.github.com/assets/1222368/476297/acf93462-b7b8-11e2-9f43-6f84e7e69212.png)
![selection_083](https://f.cloud.github.com/assets/1222368/476298/ad199cb6-b7b8-11e2-9430-41b4354d8e50.png)