added client_authentication_required() method to the validator
ask validator if client has to be authenticated
client authentication needed by default
Merge pull request #208 from masci/is_auth_required_method

Ask validator if Client has to be authenticated
Remove now obsolete doc files.
API Docs for token_expires_in. Fix #211.
Use validator.rotate_refresh_token to decide whether to replace or re-use. Fix #194.
Add installation instructions for pypi, github and a few OSs. Fix #77.
Add supported versions and platforms to docs.