Add comparison tool test runner, built on mininode

comptool.py creates a tool for running a test suite on top of the mininode p2p
framework. It supports two types of tests: those for which we expect certain
behavior (acceptance or rejection of a block or transaction) and those for
which we are just comparing that the behavior of 2 or more nodes is the same.

blockstore.py defines BlockStore and TxStore, which provide db-backed maps
between block/tx hashes and the corresponding block or tx.

blocktools.py defines utility functions for creating and manipulating blocks
and transactions.

invalidblockrequest.py is an example test in the comptool framework, which
tests the behavior of a single node when sent two different types of invalid
blocks (a block with a duplicated transaction and a block with a bad coinbase
value).