Add optional context when creating a status for a commit

Hi,

This pull request allows optional context to be given when creating a status for a commit.

From https://developer.github.com/v3/repos/statuses/#create-a-status the ``context`` is a string label to differentiate the current status from the status of other systems. The default is "default", which means any status created through ``v1.x`` will always have a context of "default". Even though the API specifies the default context to be "default", I have matched the code based for existing optional arguments by using ``github.GithubObject.NotSet``.

Cheers,
Andy