Add a main loop, move I/O code into io.go, move configuration into config.go.
Use shared.Decode24BitLength instead of manually decoding block lengths.
Fix response block decoding and checksum validation.
Add constants for commonly used byte values and use these in the signer and
the client.
The client can now talk to the old Perl signer implementation. Running
socat has been documented in README.md. Commonly used I/O code has been
moved to the shared/io.go file.
