Added support for Weibo.

Hi Brian,

I've added an oauth2 authentication module that support authentication through weibo (chinese version of twitter, only with a lot more users).

Their API is a bit tricky so to get the user information you need to follow these two steps,

* Query api to get user id
* Query api to get user information using the user id from previous request.

I've added the module to the /example and the documentation.

Anyways, I hope it's helpful and please let me know if you will consider these changes :D

/stephan