Refactor client into separate files

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.

shared/shared.go

@@ -1,3 +1,7 @@
 package shared
 
 const MagicTrailer = "rie4Ech7"
+
+const HandshakeByte = 0x02
+const AckByte = 0x10
+const NackByte = 0x11