Complete BitBucket Adapter

|Q            |A  |
|---          |---|
|Bug Fix?     |y  |
|New Feature? |y  |
|BC Breaks?   |n  |
|Deprecations?|n  |
|Tests Pass?  |y  |
|Fixed Tickets|#11|
|License      |MIT|
|Doc PR       |   |

* [x] Authentication
* [x] Forking
* [x] Handle `--org` and `--repo` arguments
* [x] `Gush\Adapter\BitbucketIssueTracker::supportsRepository()`
* [x] Paginating support

* [ ] Issues
  * [x] `Gush\Adapter\BitbucketIssueTracker::openIssue()`
  * [x] `Gush\Adapter\BitbucketIssueTracker::getIssue()`
  * [x] `Gush\Adapter\BitbucketIssueTracker::getIssues()`
    * [x] `--label`
    * [x] `--milestone`
    * [x] `--assignee`
    * [x] `--creator`
    * [x] `--mentioned`
    * [x] `--state`
    * [x] `--sort` / `--direction`
    * [x] `--since`
  * [x] `Gush\Adapter\BitbucketIssueTracker::closeIssue()`
  * [x] `Gush\Adapter\BitbucketIssueTracker::updateIssue()`
  * [x] `Gush\Adapter\BitbucketIssueTracker::getComments()`
  * [x] `Gush\Adapter\BitbucketIssueTracker::createComment()`

* [x] Labels (BitBucket does not provide labels, use meta-data instead).
  * [x] Assign type (bug, enhancement, proposal, task)
  * [x] Priority (trivial, minor, major, critical, blocker)
  * [x] Components
  * [x] `Gush\Adapter\BitbucketAdapter::getLabels()`

* [x] Milestones
  * [x] `Gush\Adapter\BitbucketAdapter::getMilestones()`
  
* [x] PR
  * [x] `Gush\Adapter\BitbucketAdapter::openPullRequest()`
  * [x] `Gush\Adapter\BitbucketAdapter::getPullRequest()`
  * [x] `Gush\Adapter\BitbucketAdapter::getPullRequestUrl()`
  * [x] `Gush\Adapter\BitbucketAdapter::getPullRequestCommits()`
  * [x] `Gush\Adapter\BitbucketAdapter::mergePullRequest()`
  * [x] `Gush\Adapter\BitbucketAdapter::getPullRequests()`
  * [x] `Gush\Adapter\BitbucketAdapter::getPullRequestStates()`
  * [x] `Gush\Adapter\BitbucketAdapter::updatePullRequest()`
  * [x] `Gush\Adapter\BitbucketAdapter::getComments()`
  * [x] `Gush\Adapter\BitbucketAdapter::createComment()` (needs API fix)                   

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