tidy up and extend validator code

[jbrunton]

• add unit tests for existing classes
• extend with other rails-like validators - e.g. numericality.

See lib/assets/javascripts/core/resources/validators for the existing validators.

Unit tests should be added to the spec/javascripts/spec/resources/validators directory. There's one example already, for the presence validator.

[jbrunton]

To run unit tests, from a checkout of the frappuccino-core repo, run

bundle exec rackup -p 4000

and then point your browser at

http://localhost:4000/jasmine

and you should see the tests all passing.