Finish Gitlab adapter

Current status of the Gitlab adapter:

* [x] Authentication
* [ ] Issues
  * [x] `issue:create` via `Gush\Adapter\GitLabAdapter::openIssue()`
  * [x] `issue:show` via `Gush\Adapter\GitLabAdapter::getIssue()`
  * [x] `issue:list` via `Gush\Adapter\GitLabAdapter::getIssues()`
  * [x] `issue:close` via `Gush\Adapter\GitLabAdapter::closeIssue()`
  * [ ] `Gush\Adapter\GitLabAdapter::updateIssue()`
  
* [ ] Comments
  * [ ] `Gush\Adapter\GitLabAdapter::createComment()`
  * [ ] `Gush\Adapter\GitLabAdapter::getComments()`

* [ ] Labels
  * [ ] `issue:label:list` via `Gush\Adapter\GitLabAdapter::getLabels()`  

* [ ] Milestones
  * [ ] `issue:list:milestones` via `Gush\Adapter\GitLabAdapter::getMilestones()`  

* [ ] Fork

* [ ] MR (aka PR)
  * [ ] `Gush\Adapter\GitLabAdapter::openPullRequest()`
  * [ ] `Gush\Adapter\GitLabAdapter::getPullRequest()`
  * [ ] `Gush\Adapter\GitLabAdapter::getPullRequestUrl()`
  * [ ] `Gush\Adapter\GitLabAdapter::getPullRequestCommits()`
  * [ ] `Gush\Adapter\GitLabAdapter::mergePullRequest()`
  * [ ] `Gush\Adapter\GitLabAdapter::getPullRequests()`
  * [ ] `Gush\Adapter\GitLabAdapter::getPullRequestStates()`