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 intent was to allow the class itself to be hidden away from view and return *only* a pointer to the base interface, so as to reduce recompilation times when the class that does the work was changed. It's not done right.