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 hate this idea that we need to keep track of the sizes - why can't the serializer do it itself, eh? - but since you have to do this you should align the comments.