Add --tags to filter by tags
Hack around Emacs bug 16121

See http://debbugs.gnu.org/cgi/bugreport.cgi?bug=16121
Fix tag filtering for Emacs 24.3.

Ert sorts the results differently on Emacs 24.3 and snapshot. This
change ignores the order.
Merge pull request #11 from rejeep/tag-filtering

Tag filtering