This commit adds the new reject protocol message added to recent versions
of the reference implementation. This message is intended to be used in
response to messages from a remote peer when it is rejected for some
reason such as blocks being rejected due to not conforming to the chain
rules, transactions double spending inputs, and version messages sent
after they're already sent.
This is work toward issue #9.
There was not much documentation about the difference between testnet and
testnet3, so make it clear that testnet is used for regression tests and
testnet3 is the public test network (version 3).
