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. 
Except I don't. The `!= 0` is here to tell me if an expiration time was, actually, specified. The check you refer to is to prevent the user from specifying an expiration time of 0.