rewrote some text as per: trac.tools.ietf.org/wg/roll/trac/ticket/155
added text from Peter van der Stok about relationship to other documents
added references to ACE charter and some credits
removed diagram and section as per: trac.tools.ietf.org/wg/roll/trac/ticket/155.
"provides no useful information here"
removed section 7 completely: http://trac.tools.ietf.org/wg/roll/trac/ticket/154
Merge branch 'master' of github.com:ietf-roll/security-threats
added references for Sybil2002 and Karlof2003, clarify text according to:
trac.tools.ietf.org/wg/roll/trac/ticket/152
fixed xml
restore section 7