Although it is possible to get the command name for each msg type by
creating an instances of the type and calling the Command method against
it, it's slightly more efficient to simply allows callers to have direct
access to the exported constants.
This is currently really useful for the reject message since callers need
to be able to examine the command type to determine whether or not the
hash field needs to be included.
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.
