added BitbucketPullrequestPoller changes source

The BitbucketPullrequestPoller polls a Bitbucket repository (merucrial
or git) for new or updated pull requests.

* master/buildbot/changes/bitbucket.py: added
added BitbucketPullrequestPoller documentation

* master/docs/manual/cfg-changesources.rst: added documentation
* master/docs/relnotes/index.rst: added release note
fix BitbucketPullrequestPoller issues
added BitbucketPullrequestPoller tests
BitbucketPullrequestPoller unittests fixed pep8 issues
BitbucketPullrequestPoller removed log messages
Merge zapster/buildbot:bitbucket-pullrequest-fix (PR #1064)

Conflicts:
	master/docs/relnotes/index.rst

+autopep8