Add new reject command.

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.
Bump protocol version to 70002.
Add tests for the new MsgReject message.

This commit adds a full suite tests for the new reject message added in
protocol version 70002 to bring the overall test coverage of btcwire back
up to 100%.

Closes #9.