[BUGFIX beta] Ensure aliased {{with}} blocks are not shared.

Use prototypal inheritance to ensure that the keywords available within
a `{{#with}}` block are not leaked outside the `{{/with}}`.

Added tests to prevent regression.

Closes #3652.
Closes #3753.

(@twokul && @hjdivad && @rjackson)
[DOC beta] Add specs for Ember.EventDispatcher#setup
[BUGFIX beta] Make Ember.Handlebars.makeViewHelper warning useful.

* Adds `ember-handlebars-compiler` to the list of packages that are
  tested.
* Uses `ViewClass.toString()` in the assertion thrown from
  `Ember.Handlebars.makeViewHelper`.

Example message before this change:

```
Assertion failed: You can only pass attributes (such as name=value) not bare values to a helper for a View
```

And after (using EAK resolver):

```
Assertion failed: You can only pass attributes (such as name=value) not bare values to a helper for a View found in 'appkit@component:api-file-link:'
```
[BUGFIX beta] Fix issue with Ember.Test.unregisterHelper.

* `Ember.Test.unregisterHelper` will currently error if the helper being
  removed is found within `originalMethods`. The issue is that
  `this.helperContainer` is not available from `Ember.Test` (it is in
  `Ember.Application`).
* `orignalMethods` has been moved into a private property of the
  `Ember.Application` instance (so that we can properly restore the
  original value).
* Added a few tests for the behavior of `registerHelper` and
  `unregisterHelper`.
Update backburner build
[FEATURE propertyBraceExpansion] Brace-expansion only in declarative form.

For performance reasons, property brace expansion is now only supported in
declarative form.  This is very unlikely to affect any users.

So the following works:
```js
var obj = { name: 'Edmure Tully', title: 'Lord Paramount of the Trident' };
Ember.mixin(obj, {
  tridentObserver: function () {
  }.observes('{name,title}')
});
```

While the following does not:
```js
var obj = { name: 'Edmure Tully', title: 'Lord Paramount of the Trident' };
Ember.addObserver(obj, '{name,title}', function () {});
```
Merge pull request #3755 from hjdivad/brace-expansion-declarative-only

[FEATURE propertyBraceExpansion] Brace-expansion only in declarative form.
Remove accepted features from FEATURES.md
Update for 1.4.0 canary
Fix Gemfile.lock
Store Router.location in container.

This was suggested in #3702, but since that PR targeted the beta branch
we decided not to make significant structural changes.

This enables the location to be looked up from the container (instead of
manually created).

Deprecates usage of `Ember.Location.registerImplementation`.
Merge pull request #3781 from kanongil/backburner-update

Update backburner build
Merge pull request #3752 from rjackson/store_location_in_container

Store Router.location in container.
Merge pull request #3775 from rjackson/fix_issue_with_unregistering_test_helpers

[BUGFIX beta] Fix issue with Ember.Test.unregisterHelper.
Merge pull request #3777 from rjackson/make_view_helper_error_helpful

[BUGFIX beta] Make Ember.Handlebars.makeViewHelper warning useful.
[BUGFIX beta] Fix default {{yield}} for Components.

Fixes #3789.

/cc @machty
Merge pull request #3804 from rjackson/fix_component_default_yield

[BUGFIX beta] Fix default {{yield}} for Components.
Remove duplicate changelog entry.
Merge pull request #3769 from pangratz/add/specs_for_event_dispatcher

[DOC beta] Add specs for Ember.EventDispatcher#setup
Merge pull request #3754 from rjackson/use_new_context_for_with_helper

[BUGFIX beta] Ensure aliased {{with}} blocks are not shared.