Refactoring of delta for unit testing

[paupino]

Primary changes are:

• Package creation is performed at the operation level and passed through as an argument to Delta::generate
• Package creation from a connection string now returns an Option so that None can be returned when a database does not exist.
• Pulled out changeset generation into a trait called Comparable. I'm not incredibly happy with that name and am open to suggestions for change. Comparable returns a PsqlpackResult so that errors can be handled (i.e. as per my plan for #10). Originally, I had this trait returning a Option<Vec<ChangeInstruction<'a>>> (quite a mouthful!) however I've now adjusted it so that it can modify the vector directly (i.e. as &mut changeset).

[paupino]

Pulled in master to resolve conflicts.