Added Validations. Added transaction_from field for the backend
Merge pull request #11 from faizkhan00/tomas-smart-property-issuance

Thank you Faiz!
Make mastercored a hard requirement for issuing assets. Make 'transaction_from' a required field. Change method signatures to be more generic, add TX51 logic for prepare_txbytes(). Use TX51 method of hex byte packing.