Add gas logic for transfer orders

[lxfind]

In #119 we added gas logic for move related orders. This PR adds gas logic for transfer orders.
It does the following:

1. Added a gas_payment field to Transfer order, representing the gas object.
2. Adjusted all related interfaces in client and authority to pass around the gas object.
3. In authority, deduct gas when executing the Transfer order.
4. Added gas_payment_object_id to Order type.
5. Adjusted all tests to reflect the change.

Mostly ready for review.
TODO: Need to add a few tests that checks the gas when sending Transfer order.

[sblackshear]

LGTM, good stuff!