Big Update, new OAuth2Client object, BC changes

This is a big update from the 0.3.0 release with a lot of usability changes:

• The "provider" services are now marked as private and have been renamed
• The public service that is made available to you is an instance of KnpU\OAuth2ClientBundle\Client\OAuth2Client and has helper methods for redirecting and getting the access token + user.
• A new oauth2.registry service was added to quickly grab any OAuth2Client object by its name

Important bug fixes, dependency updates

Removing 2.3 compat on DI - it was too much trouble

But, I did need 2.3 on router (which is fine) - before it was using something even lower,
which caused incompatibilities. Anyways, this configuration finally works with low deps set
on Composer.

First release ready for use!

[BC Breaks] The entire configuration of the bundle was totally reworked. See the configuration section for how it should be be done now.

This is still a pre-release, but unless I see big issues, I'd like to make a stable release soon.

Initial release

This is not ready to be used yet