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. 
The assert is basically checking that the result is a tes or tec class, which are the only transaction result classes stored (serialized) in ledger history. 

1. Should the assert in fact assert GTE rather than GT ?
2. If not, what other tec* code should be used instead
3. assert TER::isSerialized(ter) ?

?