An experiment client with requests-oauthlib

Hi @lepture ,

There is an experiment implementation of the OAuth 1.0a/2.0 client, which based on [requests-oauthlib](https://github.com/requests/requests-oauthlib).

The testing is poor for now. But the two examples (twitter and douban) work. Could you merge it into trunk without any document description, until I finished the refresh token mechanism and testing?

Thanks.