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.