Added YumTestCase
Use prereposetup_hook instead of postreposetup_hook.

If urlgrabber has parallel_wait() and repo has async enabled, then
doSetup will try to retrieve metadata before the "postreposetup" hook is
called.
Updated README
Make tests quieter and run on earlier versions of python
Test and fix for skip_if_unavailable: #25
Merge
Merge branch 'paulegan-yum-test'