add follow stubbing to ng-hal-mocks

you can wrap an existing hal document in the loving embrace of a mock
by calling `ngHal.mock(document);`

you can stub out followings using the `#stubFollow()` method. This
allows you to follow a chain without a whole bunch of $httpBackend
nonsense.
Merge pull request #5 from chrisrhoden/feat/ng_mock_stub_follow

add follow stubbing to ng-hal-mocks