[BUGFIX beta] Properly resolve helpers from {{unbound}}.

Prior to this referencing a helper that would normally need to be looked up with
the `{{unbound}}` helper would not work.

This meant that you could not use helpers that are stored in the container.

This commit fixes that issue.
[BUGFIX feature] Ensure that AutoLocation gets `rootURL` as an option to `create`.\nFixes #4704.\n\nAutoLocation makes decisions about replacement URLs that depend on the `rootURL` setting of the router, but the router was only providing the value of `rootURL` after `AutoLocation.create` was already called, making it not respect the `rootURL`.\n\nTo fix this issue, this commit introduces a `settings` injection type.  This type is private for now. The `root-url` setting is either `null`, the default, or a string value taken from the public API of `Router#rootURL`. `settings:root-url` will be injected as `rootURL` in all objects of `location` type.
[BUGFIX feature] Assert rootURL has trailing slash when used with AutoLocation + additional test coverage
Complete set of failing tests.
SortableMixin doesn't currently work if sortAscending and sortProperties are set at the same time.
Failing tests either copied from or based off of https://github.com/emberjs/ember.js/pull/3422
[FEATURE] Support target attribute for `link-to` helper
[BUGFIX beta] pass context to sortFunction to avoid calling __nextSuper on undefined
Invalidate previous sorting if sortProperties changes.
Merge pull request #4722 from nathanhammond/master

[BUGFIX beta] SortableMixin properties can't be set with setProperties.
Merge pull request #4717 from jayphelps/autolocation-should-have-rooturl-on-create

[BUGFIX feature] AutoLocation should have rootURL on creation + require trailing slash for rootURL
[BUGFIX beta] sync router.js; closes #4535
Merge pull request #4728 from machty/return-transition

[BUGFIX beta] sync router.js; closes #4535
The example of extending Ember.TextField with required is not relevant anymore

Since the `required` attribute is part of the recognised attributes as listed on line 27, the example about extending Ember.TextField shouldn't be about extending with `required`. 

However I am under the impression that default "coming soon" behaviour is to accept all attributes by default instead of specifying them. If that's the case the whole Extension paragraph might be irrelevant.
Merge pull request #4729 from rlopes/master

The example of extending Ember.TextField with required is not relevant anymore
[DOC release] Add reference to Ember.String.loc for {{loc}} helper so you can actually learn how to use it
Merge pull request #4733 from jayphelps/loc-doc

[DOC release] Add reference to Ember.String.loc for {{loc}} helper
Merge pull request #4714 from rjackson/properly-resolve-helpers-from-unbound

[BUGFIX beta] Properly resolve helpers from {{unbound}}.
[BUGFIX beta] Sync router.js, closes #4399, retry() bug
Merge pull request #4736 from machty/router-context-fallback

[BUGFIX beta] Sync router.js, closes #4399, retry() bug
[BUGFIX beta] Sync router.js, get rid of derelict async hook
Merge pull request #4739 from machty/omg-query-params-are-done

[BUGFIX beta] Sync router.js, get rid of derelict async hook