Add oauth2 support, update api methods, improve README, JSON related bugfix

* Add oauth2 support with examples and README usage documentation.
* Add a new api method: discussions.get_list.
* Improve README.
* Bugfix: api_methods: JSON parsing should be done with the quirks_mode option set to true.

You've done a very nice job laying the foundation for this gem, it was a pleasure to extend it and improve it.

Daniel Chcouri