Add `Matches` Validator, which checks that the value matches a regex

* Useful for confirming that string values fit a custom format.
Rename Validator Tests so they run when `rake test` is called.
Added Mathematical Validators

* Includes:
  * Greater/Less Than (or equal to) value
  * Equal/Not Equal to value
  * Divisible/Not Divisible by value
Add Accepted/Rejected Values Validators

* Checks if the value provided is in an array of values that should be
  accepted or rejected.