Ticket Support

This is the implementation of tickets, as described in https://ripple.com/wiki/Multisign, which is required for M-of-N work.

A set of integrations tests to check ticket creation, lookup and deletion will be needed and will be pushed into this request as soon as they are done. 
Why not get rid of lines 33, 39 and 39 and just say:

auto authorized = (mTxnAccountID == ticket_owner);