RateLimitHandler

Extended OAuthHandler to enable adding several access tokens to the same API object. Plus some fixes.

Provided access tokens are used selectively based on requested resource and current rate limits. The access token with most remaining requests per window for the specified resource is being selected and used when applying the authentication, before the actual request is performed. This pattern ensures the usage of available access tokens in a round robin fashion, exploiting to maximum the rate limits. 

Here's a use case I'm working on: https://github.com/ducu/twitter-most-followed#requirements