Fix #10: control over branches

Control over branches with post-receive hook URL params:

* `only_branches` - single branch name or list of comma-separated branches (e.g. http://review.example.org?service=github&only_branches=master,dev). Commits only from listed branches will be selected for code review.
* `except_branches` - works opposite to the previous option (e.g. http://review.example.org?service=github&except_branches=master,dev).