ns-3 GSOC NAT final review

ns-3 GSOC NAT semifinal review.  This code is split into three patchsets.  The first patch implements netfilter for IPv4; this represents work done before the GSOC midterm.  The second patch implements a static NAT capability on top of the netfilter patch, and was a semifinal review.  The third patch is the final review patch for the GSOC project.  

To apply to ns-3-dev (as of changeset 3a0af8a1c005 August 8 2012), first apply patch 1, then apply patch 3.