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.