
Approve a proposed escrow transfer. Funds cannot be released until after
approval. This is in lieu of requiring multi-sig on escrow_transfer

Parameters:
    from: The account that funded the escrow (type: const string &)
    to: The destination of the escrow (type: const string &)
    agent: The account acting as the agent in case of dispute (type: const
	string &)
    who: The account approving the escrow transfer (either 'to' or 'agent)
	(type: const string &)
    escrow_id: A unique id for the escrow transfer (type: uint32_t)
    approve: true to approve the escrow transfer, otherwise cancels it and
	refunds 'from' (type: bool)
    broadcast: true if you wish to broadcast the transaction (type: bool)
