[beta] controller unit-testability: stubbable `controllers` property

Removed the `readOnly`-ness of the `controllers`
CP so that it could be stubbed out for unit tests.
Merge pull request #3956 from machty/stubbable-needs

[beta] controller unit-testability: stubbable `controllers` property
[DOC beta] Document Ember.Route #controllerName and #viewName properties.