Ember.run.proxy + Ember.run.method

run-loop aware helpers for more ergonomic integration
with 3rd party asynchronous callbacks.
[DOC beta] Cleanup ember-testing documentation.

Helper Syntax Highlighting

Before:

![screenshot](https://www.monosnap.com/image/yPzfGQTdTiZXlRpNuduM2orTs.png)

After:

![screenshot](https://www.monosnap.com/image/lUF5fcv70K8sH92X3epLjpEaM.png)

registerAsyncHelper

Before:

![screenshot](https://www.monosnap.com/image/OICLOCKAoqvapb5FIecAg2QfP.png)

After:

![screenshot](https://www.monosnap.com/image/tebGA4uwb8wMhnbEzMg8hzks5.png)

registerHelper

Before:

![screenshot](https://www.monosnap.com/image/3GXahrtQ20EYs8wKbCWJw82ws.png)

After:

![screenshot](https://www.monosnap.com/image/K4sLcekKWKURKuFaExH5DYur2.png)
Merge pull request #3991 from stefanpenner/run-loop-helpers

Ember.run.proxy + Ember.run.method
Merge pull request #4014 from rjackson/fixup_test_docs

[DOC beta] Cleanup ember-testing documentation.
[DOC] Add links to ember-metal-run-proxy and ember-metal-run-method.
Merge pull request #4015 from rjackson/add_link_for_new_features

[DOC] Add links to ember-metal-run-proxy and ember-metal-run-method.