Detect also NegTokenArg packet. It doesn't have the same GSSAPI header
in same way as NegTokenInit has, which caused that packet was passed to
application instead of being processed by Waffle.
Split isSPNegoMessage into separate methods per token type.
Document detection of different tokens better.
Add separate test for NegTokenInit.
Add unit test for NegTokenArg.
Add note about NegTokenArg support.
Merge pull request #76 from AriSuutariST/spnego-tokenarg-fix

Detect also NegTokenArg packet.