implement POST /events/bulk; closes #23

We introduce an EventPack model that consists of an array of Event
objects. We don't intend to use the EventPack itself. It is merely
a means of mass creation of Event creation by taking advantage of
Mongoids relationship library.

You can provide a JSON document with an array of Event data to
create. The entire request to create a pack of Events is rejected
if any of the Event objects are not valid.
Merge pull request #24 from yobennett/event-pack-post

implement POST /events/bulk; closes #23