add flag: ACTION_VIA_SEND

:fire: :rocket: @shajith @chrsjxn @dlee

In Ember 0.9, `{{action "foo" target="bar"}}` has special behavior if `bar` is an `Ember.StateManager`. Specifically, it will call `bar.send(foo, event)` instead of `bar.foo(event)`. `Ember.StateManager` has been removed in Ember 1.0 and, with it, this special behavior. This fork introduces a flag, `ENV.ACTION_VIA_SEND`, with three allowed values:

 * `null` (the default) -- `action` uses `send` for `Ember.StateManager`s
 * `"warn"` -- `action` uses `send` for `Ember.StateManager`s, but emits a deprecation warning
 * `"1.0"` -- `action` does not use `send` and does not emit any warnings or errors