Add support for new ticket_uri field on Page events

From https://developers.facebook.com/docs/reference/api/event/ :

Name: `ticket_uri`
Description: The URL to a location to buy tickets for this event
(on Events for Pages only)
Permissions: generic access_token, user_events or friends_events
Type: string
Merge pull request #302 from wjbuys/master

Add support for `ticket_uri` field on Event