Added method "getValidationSignature" to TwilioUtils and re-routed validatRequest to use this method.

This will allow users of the helper library to more easily test Twilio request security through access to the hashed string, while still supporting the validateRequest method for those who don't need deeper integration.
Merge pull request #93 from chrismerris/getTwilioSignature

Allow library users to retrieve the hashed "X-Twilio-Signature" from TwilioUtils Library