Add a convenience method on Fauna::Resource for creating events.

Creating new events on instance event sets is a little more verbose than doing so
on custom sets. An instance of Fauna::Event can be instantiated and saved,
but it needs to be initialized with resource, set, action, and timestamp (in usecs).

On instance event sets, the 'resource' and 'set' attributes are identical. This method
lets you skip that and initialize the new event with Time instead of usecs:

pig = Fauna::Resource.find(classes/pigs/12345)
event = pig.new_event('create', 2.days.ago) # example using rails time helpers
event.save