Add script manipulation tools for use in mininode testing framework

script.py is modified from the code in python-bitcoinlib, and provides tools
for manipulating and creating CScript objects.

bignum.py is a dependency for script.py

script_test.py is an example test that uses the script tools for running a test
that compares the behavior of two nodes, in a comptool- style test, for each of
the test cases in the bitcoin unit test script files, script_valid.json and
script_invalid.json. (This test is very slow to run, but is a proof of concept
for how we can write tests to compare consensus-critical behavior between
different versions of bitcoind.)

bipdersig-p2p.py is another example test in the comptool framework, which tests
deployment of BIP DERSIG for a single node. It uses the script.py tools for
manipulating signatures to be non-DER compliant.