Extend the python mussa interface.

Extend the python mussa interface.
I needed to provide mussa load functions that take strings instead
of boost::filesystem::paths to make it easier to load analyses
from the python interpreter.

I also added code to allow directly accessing base pairs in a seq.

Lastly I disabled linking against the python interpreter for the
main mussa gui, again to simplify the problem of os x 10.4/10.5 support