Gaussian Mixture Density layer

This pull request adds a mixture density layer to PyBRAIN.  The vast majority of the work behind these files was done by Martin Felder in his commits to PyBRAIN on 9th July 2008 (commit 47aa4a6e0e034d99336b456ca2fc842951a7bd0a).  All I've done is updated his code to allow it to work with the modern version of PyBRAIN!  I have done a quick test-drive and the code appears to work but we do not have rigorous tests for this code.