Basic calendar export

What's missing:

 * Saving the generated .ical string to a file
 * Controller actions
 * Method on the model
 * Frontend stuff
Save the generated .ical file
Add #to_ical method on model
Add controller action
Add link to download the .ics file
Add event categories and text to exported file
Merge pull request #40 from BatchZero/ExportCalendars

Exporting .ical calendars