create unsigned tx and remember sig_hash for each input

allows the wallet to create transactions spending outputs for which
we don't have the private key, so we can't sign them.
instead, remember the signature hash that needs to be signed, so
they can easily be signed later.
#create_tx and #assemble_tx commands

allows for easy creation of transactions without parsing any bitcoin-specific things
and without the need to reveal the privatekey to the node
Adding 'Compatible with...' section to README.rdoc