Feature/sentry cleanup

So none of the signature code is actually used, so all the hmac-sha1 code went away.  

The auth_header code simplified down to what amounts to a sprintf call that just encodes the DSN username.  The DSN password isn't actually used anywhere.

I also removed a bunch of attributes from the SentryMsg struct as they aren't necessary.

This code will need new tests against the sentry output, but I've verified that it does work in a full stack.