first cut of gssapi rfc

Hi there,

I've made an attempt at drafting the new gssapi mechanism rfc.  As suggested by Pieter, I've used 25 as a guide.  This doesn't spec out the GSSAPI (I've made reference to the gssapi rfc).  It's very similar to the ZMTP Curve RFC.  Let me know if this fits the formula or if there are any suggestions for improvement.

Thanks!