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. 
std::pair <char const*, char const*> should be a struct.  "anonymous" elements like first and second lead to error.  Remember - you can still use uniform initialization!  :-)