Adding toString implementation for RuleViolation and GroupViolation

[Diagoras]

This is a shot at addressing issue #78, adding a nice toString implementation for Violation. I'm a little concerned about the recursive aspect though, since a GroupViolation can contain another GroupViolation. Would you prefer that was handled with an @tailrec annotated method or ScalaZ's Trampoline?

[holograph]

Since this is intended for debugging, I wouldn't necessarily worry about performance. I generally like this solution, and if performance becomes an issue, @tailrec is definitely the way to go -- I'm trying to keep dependencies to a minimum, zero if possible.