Stop functionality has issues when tested

See https://github.com/spira/angular-http-progress/blob/v0.1.0/test/test.ts#L153 in #1 the test has been skipped. With a concrete implementation the functionality of the `.stop()` method needs to be tested as I suspect there is something wrong with how the stop/restart flow is supposed to work. For now I have skipped the test.

This issue stops a major first release of this module.
