token.key should be a Buffer, but stored as hex

token.key is used by Hawk as the HMAC key. If its
a Buffer, it uses those bytes, if its a string Hawk
converts the it to a Buffer with UTF8 encoding. We
want Hawk to use the "raw" bytes. This was discovered
in #114
Merge pull request #114 from dannycoates/hawkKey

Clarify splitting {session,keyFetch}Token into id and reqHMACKey