Implement oauth2/invalidate_token

Here's my first attempt on implementing [POST oauth2/invalidate_token](https://dev.twitter.com/docs/api/1.1/post/oauth2/invalidate_token). I'm pretty sure I missed a few things from your lecture last Sunday, but I think I got most of them right.

When I implemented `Twitter::Token`, I wasn't really sure where to inherit from, so I settled with `Twitter::Base`. Let me know if I still need to adjust a couple of things.

Lastly, I apologize for being a few days late with the pull request. I've been swamped with work all week and I was really only able to dedicate time for this just now.

Cheers!