Added Unit Tests for AuthenticationImpl

To show that directly unit testing ice interfaces is fairly straightforward, I've created unit tests for the Authentication interface.

Right now one of them is failing, since login() doesn't check for a callback interface yet, but that's intended. Somebody go fix login() ;)

To make unit testing possible, I've used gmock to mock a few core Ice classes and have them return dummy values. Those should be reusable for other tests, so I put them into a separate file in "tests/include/IceMocks.h"