Split Registration into Server and Direct

[judgej]

The Registration class is going to get pretty big once SagePay Direct is added. Instead, it should be split into Direct and Server classes. We already have a Shared class for the shared services.

A Common class will be needed for the functionality that all three share.

Registration can inherit Server for backwards compatibility, but otherwise be deprecated.

[judgej]

This has been split now into Direct, Server and Shared.

Since Direct and Server extend Shared, and Shared extends Common (and it the only extension), I think Common is a misnomer. It should be a service abstract class (perhaps ServiceAbstract) and so should be renamed.

[judgej]

Please note that Register has been kept but deprecated. It expends Server, so offers legacy support (this re-factorisation should not break applications that use the library already - I'm trying to be careful about that). But tests - damn I need to write some tests.