Implement ability to sign transactions with multisig address.
This should be done in several steps:
1. Create TX with `wallet transfer --out <file>`
2. Sign TX with `wallet multisign sign --in <file> --out <file2>`.
3. Repeat 2 for every party.
Input file contains transaction with possibly incomplete
set of the signatures. Output file will contain the same tx
with updated signature set.
When --rpc flag is provided, result transaction is sent
via `sendrawtransaction`.
