Add an auth_reversal method for Litle which is the preferred way to reverse an authorization.

* Added certification and unit tests for auth_reversal.
* Fixed certification tests to always use a random string of length 24 which is required by the XML schema
Remove auth_reversal from the public interface and implement it in #void

To maintain the same interface as the other gateways #void now detects what kind of transaction it is voiding based on an indentification that is composed of the transaction-id and the kind of transaction.  If it is voiding an authorization it will perform an auth_reversal, otherwise it will perform a void. This means Response.authorization will also now always contain this composed identification.
Merge pull request #5 from LessonPlanet/litle/auth-reversal

Add an auth_reversal method for Litle which is the preferred way to reve...