Updates to the README:

* Say that OMPI is now MPI-2.2
* Add Cygwin as a loosely-tested platform
* Remove mention of Myrinet GM
* S/QLogic/Intel/g
* Remove mention of --with-elan, --with-sctp, and --with-udpal
  configure options (no such components exist in the v1.7 tree)
* Remove mention of -with-cray-pmi-ext configure option (it's not in
  "./configure --help" output)
* Remove mention of --enable-sensors, --enable-heartbeat, and
  --enable-orte-progress threads; the first two are dead code, the
  last does not show up in "./configure --help" output
* Minor other text teaks