fix PullRequests merge and decline, and posting comments

PullRequests: was missing content-type
Merged in sstok/bitbucket-api/fix-PullRequests (pull request #6)

Fixed Content-Type for `PullRequests::accept` and `PullRequests::decline`.
make tests green again
implemented simple priority for listeners
Merge branch 'feature/listeners-priority' into develop
renamed RequestListener to NormalizeArrayListener and added ashort description for this listener.
Repositories::all returns MessageInterface
fixed docblock in Client.php for lastResponse and lastRequest
Allow setting custom Request and Response in HTTP client.

This should allow an easier integration into 3rd party software.
version bump
Merge branch 'release/0.5.0'
Merge tag '0.5.0' into develop

0.5.0