Misc Field/validation changes

- Bumped required version of Concur to 0.2.4 for acccess to inheritance chain details via <Constructor>.__mro__
- Bumped required version of validators to 0.2.3 for some validation error fixes
- Default error messages are now collated by the Field constructor instead of having to be collated manually for each Field subclass
- Removed formatting of validation error messages from fields.js - params are now passed to ValidationError instead
- Ensured single message ValidationErrors are being created with a code so they can be customised
- Made emptyValues a property of Field.prototype so it can be overridden by subclasses if necessary
- TypedChoiceField#coerce can now return an arbitrary value