Controller events hash

This PR moves where controller action handlers are expected to be defined from
the controller itself to the controller's `events` object.

This makes controller action-handling consistent with how `Ember.Route`
action-handling works.

The previous behavior continues to work but will emit a deprecation warning.

Relies on #2888 (`mergedProperties`) being pulled first.

Questions for to consider for this PR or future changes:

  "Should we apply this `events` hash to views as well?"
  "Should we rename `events` to `actions` (with deprecation)?"
  "Should we rename `send` to `sendAction` (with deprecation)?"

Future TODOs:

  * Extract a mixin (this will be easier after deprecated functionality is removed)
  * And then, eliminate duplication of `send` code with router microlib code