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. 
I think the intent is to put the class declaration in the .cpp file so that clients don't see the declarations (a style I approve of) but it wasn't done right (the class declaration is still in the header).