Added gzip filter

* Added gzip/gunzip enumeratees, with tests
* Added gzip filter, with tests
* Added two new iteratee concurrent facilities, a joined
  iteratee/enumerator, and a partial run of an enumerator that returns a
  future with the iteratee result and an enumerator that links up to the
  enumerators remaining input.  With tests.
* Documented how to use the gzip filter in Scala and Java, and added it
  to the 2.2 highlights.