Problem: Hard to check message type without destroying it

Solution: Implement is_class function to check signature and id.

In more details, if I can receive the same way various different
messages, I need to branch based on the message type so I can properly
reply. Easiest way would be to try decoding message, but that will
destroy the message, so implementing is_class function, that checks for
valid signature and message id without destroying the message.
Merge pull request #154 from miska/master

Problem: Hard to check message type without destroying it