nose and modules with optional library deps

[Elrond]

During unit tests nose imports every ``*.py`` it finds in the tree.
It does so to search for unit tests there. If the import fails,
this gives an error in the final unit test results.
For GMG this can fail, because some modules import optional
libraries (video for example).

Currently known options:


1. Add hard dependencies again for all optional libraries. Not
   nice, but would solve the problem at hand.
2. Rewrite the affected modules to somehow gracefully handle the
   situation and not error. Not really nice, but acceptable.
3. Change ``runtests.sh`` to call nose in a way, that tests are
   only searched in ``mediagoblin.tests``. This seems the nicest way.