fixes bug 885871 - No such file or directory: favicon.ico, r=rhelmer

This changes so that offline compression is by default DISABLED. That makes
running the whole test suite about 10 times slower.
Alternatively a developer (who runs tests often and wants to run them fast)
is instructed in settings/local.py-dist how to change it.

Also, this patch changes how the favicon test is run. Instead of relying on
``collectstatic`` having been run, it instead uses a temporary directory
thus making sure that running ``./manage.py test`` doesn't affect any of
the static files created.