Added permute method to HRR.py; created HDM.py

Added a permute method to the HRR class in the HRR.py file. Seems like an obvious thing to add to the HRR toolbox, and worth adding to your master copy.

I've also added HDM.py to my branch, but HDM.py isn't ready for prime time (in my opinion). It's an implementation of a BEAGLE/DSHM thing as a substitute for ACT-R's DM and seems to be working, but we're continuing to test it. It was made by modifying DM.py, so it probably contains a bunch of code that isn't doing anything and could be deleted, but I'm a little afraid to delete things I don't understand.