Improvements in flexibility and user-friendliness.

- Made it so that the '%s' in error strings is optional so non-formatted strings can be used for errors where calling a field by name is unnecessary.
- Added an alphanumeric validator for convenience.
- Changed the default message of the required validator from '%s is required.' to 'This field is required.' because any sensibly laid out form already makes it obvious what field an error refers to.