SCM Web Hooks for reviewboard : Phase 1

Generate url infrastructure for hosting services so that they can be called to
access web hooks. Added repo_urlpatterns to the service base class. Modified
registration and unregistration of hosting services to add and remove
urlpatterns respectively.

Testing Done:
Wrote test case HostingServiceUrlPatternTests in hostingsvcs.tests.py. I've
tested the addition and deletion of url patterns during registration and
unregistration, respectively. Added tests for cases when the url_patterns are
not defined. Also tested adding bitbucket url patterns (see screenshot). All
tests have passed. :)

Reviewed at https://reviews.reviewboard.org/r/5510/