Fullcalendar

This pull request introduces the FullcalendarEngine, which is a rails gem for the [http://fullcalendar.io/](fullcalendar) calendar written in JavaScript. This calendar will be used to visualise bookings.

A booking now has a FullcalendarEngine::Event. Booking's attributes title and description is now replaced with Event's equivalents. Event also has time information, color and repeat, but these are not used yet.

Fixes #47
Fixes #48
Fixes #53
Also #56 and #58, but they are already marked as fixes in commits.