Finish Gitlab adapter

Current status of the Gitlab adapter using `m4tthumphrey/php-gitlab-api` as mentionned in #2:

* [x] Authentication
* [ ] Issues
  * [x] `issue:create` via `Gush\Adapter\GitLabAdapter::openIssue()`
  * [x] `issue:show` via `Gush\Adapter\GitLabAdapter::getIssue()`
  * [ ] `issue:list` via `Gush\Adapter\GitLabAdapter::getIssues()`
    * [ ] `--label`
    * [ ] `--milestone`
    * [x] `--assignee`
    * [x] `--creator`
    * [ ] `--mentioned`
    * [x] `--state`
    * [ ] `--sort` / `--direction`
    * [ ] `--since`
  * [x] `issue:close` via `Gush\Adapter\GitLabAdapter::closeIssue()`
  * [x] `issue:assign` via `Gush\Adapter\GitLabAdapter::updateIssue()`
  
* [x] Comments
  * [x] `Gush\Adapter\GitLabAdapter::createComment()`
  * [x] `Gush\Adapter\GitLabAdapter::getComments()`
  * [x] Issues
  * [x] MRs

* ~~Labels~~ **Not supported**: Gitlab does not provide an API for labels
  * ~~`issue:label:list` via `Gush\Adapter\GitLabAdapter::getLabels()`~~

* [x] Milestones
  * [x] `issue:list:milestones` via `Gush\Adapter\GitLabAdapter::getMilestones()`  

* ~~Fork~~ **Not supported**: Gitlab does not provide an API for forking repositories. It only provides an **admin API** to edit projects' relationships.

* [x] MR (aka PR)
  * [x] `Gush\Adapter\GitLabAdapter::openPullRequest()`
  * [x] `Gush\Adapter\GitLabAdapter::getPullRequest()`
  * [x] `Gush\Adapter\GitLabAdapter::getPullRequestUrl()`
  * [x] `Gush\Adapter\GitLabAdapter::getPullRequestCommits()`
  * [x] `Gush\Adapter\GitLabAdapter::mergePullRequest()`
  * [x] `pull-request:list` via `Gush\Adapter\GitLabAdapter::getPullRequests()`

* [x] Handle `--org` and `--repo` arguments