Replace-by-fee regression tests

First get version v0.5.0 of the python-bitcoinlib library. In this directory run:

git clone -n https://github.com/petertodd/python-bitcoinlib
(cd python-bitcoinlib && git checkout 8270bfd9c6ac37907d75db3d8b9152d61c7255cd)

Then run the tests themselves with a bitcoind available running in regtest mode:

./rbf-tests.py