Add Starred API, revised Watchers API and Improved Docs

* Added Starred API for get starred repos, star an repo and unstar an repo for authenticated users.
* Revised Watchers API using a new endpoint as shown in the [GitHub API](https://developer.github.com/v3/activity/watching/).
* Improved Documentation | Added incomplete Activity documentation for Starred and Watchers API.
Changing the return value of an existing method is a BC break