sendmany attempt

I needed to have sendmany like in bitcoin-qt, but with change address and fee control. Of course electrum is always my go-to client for custom tailored transactions without the perils of creating them by hand, so I took a stab at it.

I have not tested actually submitting the tx, but I reviewed a bunch of them from mksendmanytx and it looks very sane, as it should as the change is minimal.

I thought about piggybacking on payto/mktx, as the optional 'label' described in the help text does not seem to be processed anywhere, but I only did a quick search for it and didn't want to change the existing command expected behaviour.