Valutec phase 2

Notable additions: 
* 3 responses are now being parsed
  * Restaurant Sale response (Restaurant Sale allows adding a tip)
  * Card Balance response
  * Void response
* ValutecClient#authorize has been updated to perform the actual charge. Valutec does not offer an auth/capture workflow.
* ValutecClient#settle returns a standard response of successful? true instead of making a call to Valutec