Improved documentation

I've tried to make a better description of what's going on in the module.

I also suggest changing the "Description" on the github page from "OAuth support for Mojolicious" to "OAuth2 support for Mojolicious".

* A lot more clickable links in the text to third parties
* https://github.com/jhthorsen/Mojolicious-Plugin-OAuth2/tree/doc#references
* Split SYNOPSIS into four parts
* Move CONFIGURATION to SYNOPSIS
* Add example plugin() code to SYNOPSIS
* https://github.com/jhthorsen/Mojolicious-Plugin-OAuth2/tree/doc#custom-connect-button
* Moved DESCRIPTION above SYNOPSIS since the SYNOPSIS is rather long now
* Add more links to the providers under "Configuration"
* Add more details to https://github.com/jhthorsen/Mojolicious-Plugin-OAuth2/tree/doc#get_token
* Fix links to "get_token" and "get_authorize_url"
* Fix =head1 for AUTHOR and LICENSE

...and I added myself to the AUTHOR section.