[WIP] OAuth2 Subscriber

Salut Eric,

this is a very rough first implementation of an OAuth2 subscriber, that I've been working on since yesterday. It currently only supports the Authorization Code Grant and the Refresh Token Grant, the PHP Doc is not cleaned up, the subscriber itself is not in place yet, and so on :). At least everything is fully tested!

I have published the PR already because of two reasons:
- Would you include an OAuth2 subscriber in your HTTP Adapter at all? :) Or would you rather recommend making it a separate extension in a separate repository?
- Either way, if you can look past the obvious stylistic flaws which are in the code at the moment, I would really appreciate your feedback on this.
- And at last, on my machine, the `Ivory\Event\OAuth2\Configuration` class gets no code coverage although the tests are covering it. I wonder if Travis gets it right.

Cheers
:christmas_tree: Jérôme