Added minimum-stability to composer.json as stable

* Also added unstable branch alias as 2.3-dev
Solved #70. Added new parameter configuration

* Added parameter workers_name_prepend_namespace with true as default
* Added documentation of feature
* Fixed some phpcs
* Added some new files, as first Changelog ( empty at this moment )
* Added contributing file with its content
* Refactored function getFileClassNamespace in GearmanCacheWrapper
Added number of downloads counter
Added unstable tag badge