Avoid failure if you delete the bitbucket_rest_api in Gemfile

The bitbucket_rest_api gem is need only in issue_tracker bitbucket. If
you optimise your Gemfile by delete it errbit not work. But he need to
work.

Fix #558
Update changelog with starting 0.2.1 and bug fixes

[ci-skip]
Update Changelog and contributor to start 0.3.0 version

[ci-skip]
Merge branch 'master' into mongoid-3

Conflicts:
	CHANGELOG.md