Block wires sending invalid pieces
Update doc
Keep a list of source wires
Implement bad piece strikes counter, #59
added a test that verifies blocking on bad piece
destroy the engine at the end of the test
unchoke the peer to speed up tests
fixed block method and added tests to verify blocklist
Merge pull request #64 from mafintosh/auto-block

Block peers that send 3 bad pieces within 2 minutes