Refine Mastercoin information and fix symbol "MSC"

The symbol used to represent Mastercoins is "MSC".

Source:
https://github.com/mastercoin-MSC/spec#master-protocol--mastercoin-terminology

A reference to the "Exodus" address is not limited to Mastercoins (as token), but used protocol wide as transaction marker at this point. This PR is furthermore intended as a refinement of Mastercoin's encoding schemes.

Source:
https://github.com/mastercoin-MSC/spec#class-a-transactions-also-known-as-the-original-method
https://github.com/mastercoin-MSC/spec#class-b-transactions-also-known-as-the-multisig-method