Refactor generate_slug into a mixin.

Refactor the new generate_slug into a mixin to
allow it to be used for collections also.
Use GenerateSlugMixin for collections.

Use the new way of generating slugs also for collections.
Also drop the dummy_db arg to check_collection_slug_used.
Better docs for GenerateSlugMixin.
Really removing nosetests things now!  all assert_whatever removed
Teach runtests.sh the -n arg.

If you give "-n 8" to runtests, it thinks, it got
"-n" -- a nice option, ignored.
"8"  -- Oh! a single test named "8" to run, so no need to
        run all the GMG tests.

Well, that's not what we want. So runtests now knows about
-n taking an arg.