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. 
@sublimator: Your comments weren't deleted - there was a force-push when I rebased this branch against the latest develop.

Re: the `assert`, I believe that it should be `>= 100` *but* I can't just up and make that change without carefully considering the implications. I'll update the branch accordingly.