createEvent support

Intercom introduced a new function to track Events, see:
http://docs.intercom.io/filtering-users-by-events/Tracking-User-Events-i
n-Intercom. The events API endpoint makes it possible to track events
via the API, see: http://doc.intercom.io/api/#events .

This commit implements the createEvent method to utilize the events API
endpoint in order to track events to Intercom.