Added checks to Message originator getter / setter

Added strlen check to the originator getter / setter due to the originator not being allowed to have a string over 11 characters. Over 11 and all you receive is a 400 Bad Request message. Only Mention I can see of the character limit is on http://developers.esendex.com/APIs/REST-API/messagedispatcher and if you're using the SDK you may never even look at these pages in the documentation!