Adding tests and code to allow subclasses of `oauthlib.oauth1.Client`
Pass any additional `kwargs` to the `client_class`...
Pass the `client_class` to `requests_oauthlib.OAuth1` and any additional `kwargs`
Merge pull request #113 from al-the-x/master

Support using subclasses of `oauthlib.oauth1.Client` with `requests_oauthlib.OAuth1`