Refactor claims validation to reduce complexity

Code Climate didn't like the structure of my `validate_claims`
implementation. Refactoring caused the class to grow beyond RuboCop's
limits, so the functionality was pulled out into a module.

The result is pleasing, I think.
Added a note about the resulting callback URL
Merge pull request #5 from ausaccessfed/feature/minor-tweaks

Some minor tweaks