s/cStringIO/io for Python 3 compability
Remove coredefs import in messages.py

coredefs.py does no longer exist. It has been removed with
28518d296cfa996fb9b95e6b0deafcf98bd93caf
Add missing (MIN_)PROTO_VERSION definitions

They got lost when coredefs.py was removed, but are still used by
messages.py.
Merge pull request #2

0891874 Add missing (MIN_)PROTO_VERSION definitions (Florian Schmaus)
2a2327c Remove coredefs import in messages.py (Florian Schmaus)
bc76c9f s/cStringIO/io for Python 3 compability (Florian Schmaus)
Encode -1 as OP_1NEGATE

See BIP62