[BUGFIX release] Fix #3681.

`Em.arrayComputed` is not presently compatible with lazy CPs. There are a few
issues.

1.  To keep refs up to date, observers stay around as long as the object does.
2.  Those observers can repopulate the cache, but when they do so they fail to
    add dependent keys when the cache was empty.

This commit works around 2. by not removing them in the first place.
Essentially the semantics for `ArrayComputedProperty`s differ from standard
`ComputedProperty`s with regards to laziness.

These observers need to be managed better.  Probably the right thing to do, now
that `recomputeOnce` is no longer necessary, is to:
  1. Fix `Em.computed.sort` to not use `recomputeOnce`;
  2. Fix `Em.arrayComputed` to return a proxy to the cached value;
  3. Teardown array observers when the deps count hits 0.

This will give reduce computed properties semantics much closer to regular
computed properties.  There are two important points to note about this
suggestion:

1.  The cached values would still update synchronously.  This can be improved
    with a lazy array implementation and @wycats's skiplist strategy.
2.  The semantics of an array computed would change such that existing refs
    would only be kept up to date as long as the cache had not been invalidated.
    This is probably fine: the current behaviour is not promised in the
    documentation anywhere and it would be pretty bad practice to keep
    references around rather than `Ember.get`ing them.
[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] 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.