feat(ngMock.$httpBackend): added support for function as URL matcher

It's now possible to pass a function to match the URL in $httpBackend mocked
expectations. This gives a more sophisticate control over the URL matching
without requiring complex RegExp mantainance or the workaround of creating
an object with a `test` function in order to mimic RegExp interface.

This approach was suggested in [this
thread](https://groups.google.com/d/msg/angular/3QsCUEvvxlM/Q4C4ZIqNIuEJ)

Closes #4580