Add extra docs to Github package; 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.