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 would write
```
// Count the ticket towards the account reserve
auto const newReserve (mEngine->getLedger ()->getReserve (
    mTxnAccount->getFieldU32 (sfOwnerCount) + 1));
```