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 still have trouble with this construction over =.  I read this first as a function call.

We use () so many places - for arithmetic, for function calls.  = on the other hand we ONLY use to assign a value to a variable - so it's totally clear what it does.  Why replace the classic old int i = 0; we've had forever and is unambiguous with int i(0);  ?

Just a whine... :-D