Add a 'masked' extension to handle masked input

This doesn't actually seem to be substituting MaskedCovarianceMatrix
instances like it's supposed to, but I thought I'd post my work in
progress so others could chime in.

See the associated mailing list discussion for more details:

http://sourceforge.net/mailarchive/forum.php?thread_name=20130325121945.GI18933%40bio230.biologie.hu-berlin.de&forum_name=mdp-toolkit-users