Encoding specifies a way to encode/decode NEO messages and packets.
Current way of how messages were encoded is called to be 'N' encoding.

This patch:

- adds proto.Encoding type
- changes MsgEncode and MsgDecode to be methods of Encoding
- renames thigs that are specific to 'N' encoding to have 'N' suffix
- changes tests to run a testcase agains vector of provided encodings.
  That vector is currently only ['N'].