Support multi-sig accounts in genesis files

[batconjurer]

Currently genesis only supports accounts owned by a single key. This should be expanded to support multi-sig accounts.
This will require some thought as currently balances are assigned to single keys from which they can be transferred to established accounts. In short, all balances initially belong to implicit accounts. There is no good equivalent for multi-sig accounts, so this needs to be brainstormed.

Secondly, multi-sig txs should be supported in the genesis transactions and the client tooling should handle this. This is more straightforward.