Add docs to Github; light refactoring; bug fixes.

* Rename `Package` and `GithubObject` classes to `AbstractPackage` and
`AbstractGithubObject` respectively.
* Deprecated `Package` and `GithubObject` classes.
* Fix bug in create tag API.
* Fix bug in `Teams::chekRepo` API (missing repo owner name - this is
technically not backward compatible, but the old method would not
anyway)
* Added support for releases.
* Added more examples to documentation.
Fix merge conflict.
Removed stray file.
Merge pull request #295 from eddieajau/Github

Add extra docs to Github package; light refactoring; bug fixes.
Merge https://github.com/joomla/joomla-cms/commit/9aa14cfa34731d3b6a70d3f50bf7293e109206e5
Adding container->exists method and test
Updated as per dongilbert's comments
Merge pull request #309 from wilsonge/patch-2

Fix E_STRICT notice in FilterInput
Merge pull request #310 from piotr-cz/feature_DI

Adding Container->exists method and test