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. 
The quorum threshold indicates the minimum required weight that the sum of the weights of all signatures must have before a transaction can be authorized - it must be at least 2.