[DOC] Add docs for ArrayProxy content observer methods
Merge pull request #4752 from davidgovea/docs-add-arrayproxy-observers

[DOC] Add docs for ArrayProxy content observer methods
Added `withKeyCode` option to action helper

For actions that are fired on `keyUp`, `keyDown` or `keyPress` an option called `withKeyCode` is now available to use. It ensures that the action is called only when the given `withKeyCode` value matches the keyCode on the event.

For example:

    <input type='text' {{action 'show' on='keyUp' withKeyCode='13'}}>

This action will only fire if the event is a `keyup` and the `keyCode` is 13.
Merge pull request #5000 from crismali/enter-action-key

Added `withKeyCode` option to action helper
Cleanup of incorrect feature flag in #5000.
Remove duplicate feature flag.
[FEATURE ember-metal-injected-properties] Added `InjectedProperty` descriptor

This adds a new `Descriptor` type for defining a property that does a
lazy lookup on an object's container. There is no public API addition
since this kind of property is easily abused. Instead it will be used by
internally `ember-runtime` API additions, where validation can happen.
Added `inject` namespace and injected property creation/validation

This adds a new property to the root namespace that provides computed
properties for container lookup that are enforced by type. This is to
disambiguate between other top level properties that are very similar.
Any ember module can create a new per-type injection helper by calling
`createInjectionHelper` and providing a mixin-time validation function.
Added the `inject.controller` injected property helper

The helper restricts controller injections only to other controllers, so
it cannot be used on routes, views, etc.
Added `Ember.Service` and the `inject.service` injected property helper

This adds the base class for Ember 'services' and an injection helper
that makes referencing container-singleton services from routes and
controllers simple.