Update for Merge Requests API:

- getting MR in different states
- `labels` property for MR model
Merge branch 'master' of github.com:m4tthumphrey/php-gitlab-api

Conflicts:
	lib/Gitlab/Api/MergeRequests.php
	lib/Gitlab/Model/MergeRequest.php
	lib/Gitlab/Model/Project.php
Merge pull request #36 from arturf/master

Update for merge request api