Extract a method that just converts a string to a Clever class name

This is used for converting uri strings to classes, and will soon be useful for converting the first part of the event type to classes.
Also add 'push' => Event mapping.
Support retrieving events for a district
Update District#events to use the now-linked events resource
Use VCR's allow_playback_repeats setting instead of duplicating requests in the cassette
Merge pull request #8 from thinkthroughmath/events_support

Events support