Container throws if injection registered for already instantiated type.

Prior to this commit, the container would happily allow you to register an
injection for a type that had already been instantiated. This led to
confusing-to-debug situations where an injection is registered but a
container-created instance doesn't have the injection applied.

Also covers factoryInjections.
[BUGFIX beta] Ensure context is unchanged when using keywords with itemController.

Prior to this change the following code would bind `this` in the
template block to the itemController's content (even though the keyword
form of `{{each}}` is used).

This change sets the child view's `_context` property to the current context when using
the keyword form of `{{each}}`, and a couple of confirming tests to
demonstrate using `itemController` specified in either the
`ArrayController` or the template directly.
[BUGFIX beta] Remove old reference to invoker.

As of 6a8137902da56b8b1060fcd0bdb748d5e7ae0092 we no longer use a custom
`invoker`, and the logic was moved into `registerObserver` directly.
[BUGFIX beta] Update the yuidoc paths to the new packages_es6 directory.
fix readme inconsistencies
Merge pull request #4693 from bmac/fix-doc-build

[BUGFIX beta] Update the yuidoc paths to the new packages_es6 directory.
[DOC release] fix sample code inconsistencies
[BUGFIX beta] RSVP errors go to Ember.onerror if present
Merge pull request #4597 from knownasilya/typo

[DOC release] comment examples and consistencies
Merge pull request #4696 from knownasilya/readme

Fix README inconsistent quotes
[DOC release] fix documentation for childViewDidChange for number added
Merge pull request #4686 from rjackson/fix-removed-invoker

[BUGFIX beta] Remove old reference to invoker.
[DOC release] Add yui doc @since tags to methods and properties added after 1.0.0
Merge pull request #4697 from jhsu/fix-doc-var-name-for-childviewsdidchange

[DOC] fix documentation for childViewDidChange for number added
Merge pull request #4694 from bmac/since-docs

[DOC beta] Add yui doc @since tags to methods and properties added after...
[BUGFIX beta] Better error message for rejected routes.
[BUGFIX beta] Does not disregard currentWhen when given explicitly.
FIxes #4688
[DOC beta] Fix warnings generated by yuidoc.
[DOC beta] Update the FEATURES.isEnabled docs so it doesn't look like a method on the Ember object.
Merge pull request #4703 from bmac/cleanup-yuidoc-warnings

[DOC beta] Fix warnings generated by yuidoc.