[pymvpa] dissimilarity stuff

[James M. Hughes]

Hi all,

I just pushed into my branch an implementation of some stuff from  
Kriegeskorte's representational similarity analysis paper.  Well,  
actually it's a generic (unoptimized) implementation of a  
dissilimarity matrix, along with a "DSMDatasetMeasure" (i.e.,  
dissimilarity dataset measure) for using dissimilarity matrices in  
searchlight algorithms, etc.

The code's not pretty and might break things, so please have at it!

Cheers,
James.