Feature/token session support

Adding basic support for service providers that implement the [OAuth Session 1.0 draft 1](http://oauth.googlecode.com/svn/spec/ext/session/1.0/drafts/1/spec.html) specification.

This allows clients of service providers like Yahoo to easily refresh the access token without having to go through re-authentication. This addresses [Issue #4](https://github.com/mrjones/oauth/issues/4).

Updates to the API:
- Added `RefreshToken` method.
- Added `AdditionalData` key/value map to `AccessToken`