Changes to core library

Hi Willem,

I've been doing a lot of pulling things around in your adyen codebase, and wanted to open a pull request.  I've removed a lot of the Rails-specific stuff and pulled out some code into smaller classes to try to better encapsulate things.  I've tried to keep things backwards compatible for the core functionality, although I may have missed some things (plus removing the Railtie will break things for Rails users....)

The change history is a bit messed up as I started on an engine in here and then moved everything outside and tiied things up.  I can rebase everything to remove that history but wanted to at least make you aware of what I've been doing up to now.  Also, I've already upped the version file to 2.0.0.

I've also got the majority of the works for a Rails engine that I will push up to github soon.  I'll send you the link once I've got it in place but need a new version of the Gem without the Railstie, etc. to carry on.

Thanks,
Neill