circuit transfer add atomic swaps

[barryWhiteHat]

Add conditional payments to the circuit so we can do atomic swaps.

1. Create a extra fields in the signature dep_prev_to , dep_prev_from, dep_prev_amount, dep_next_to, dep_next_from, dep_next_amount for both the payment beore and after.
2. If these vaules are zero do nothing
3. If they are nonzero then the transaction is only valid if the following

tx[i -1].from == dep_prev_from
tx[i -1].to == dep_prev_to
tx[i -1].amount == dep_prev_amount

tx[i _1].from == dep_next_from
tx[i +1].to == dep_next_to
tx[i +1].amount == dep_next_amount

[GuthL]

@vaibhavchellani Are you on it? Or would you want me to do it?