Condensed the url_for calls in Calendar init #11

The url_for is now using the shortcut paths such as
bookings_path(:format=>:json) to generate the urls for the Calendar
class.
Adde overthree scope to Person model #10

The overthree scope will allow us to check the number of spots
avaiable correctly since only people over 3 should be counted in the
number of open spots.
Added a check for spots available before updating a booking #10

Added a before_update check to make sure that spots are available
before updating a booking.
Fixed issue with creating a booking  #12

Fixed an issue where you could not create a booking due to the num
spots available check. Now we check to ensure that the num_spots
param is not nil and that the people list are not nil before trying
to calculate. When a booking is new these will be nil and we just set
the number available to 0.