Change default urlRoot to '/__testacular/'

Reopening vojtajina/testacular#268 against the master branch instead of stable.

Looking through issues & google groups, there's a pain point when testacular captures the root path for itself.

I don't see any use case where the default urlRoot _should_ be '/' -- and quite the opposite is true; every application will have _something_ bound to '/' -- so, it seems more logical to bind the test runner to something that won't collide with the code under test.