Passay #10.

Rename RegexRule to IllegalRegexRule.
Add AllowedRegexRule.

See https://code.google.com/p/vt-middleware/issues/detail?id=207

Every other rule has a single ERROR_CODE, this one would have to have two since the messages are different.
RepeatCharacterRegexRule extends IllegalCharacterRegexRule, if it's base class changes it could be improperly configured by flipping this boolean toggle.
I understand the principal of code reuse here, but I'm still leaning towards consistency and parity with the other rules.