Subclassed btcointx.CKeyBase to create a privkey type
that uses our signing code.
These are then used to allow signing of inputs in
transactions owned by our wallet.
Created a PSBTWalletMixin to perform the above function,
and added it to our three wallet types.
Wrote a detailed creation, sign and broadcast test
for a psbt using utxos from within and outside the wallet.
Detailed tests across all 3 wallet types for psbt
Tests cover also mixed inputs of different
types and owned/unowned. Direct send now exported to be used
in tests rather than only script usage, also supports returning
a tx object rather than only a txid.
Adam Gibson