Added hypg dist and -nw flag.

I added the hypergeometric distribution in a file called hypg.py.  This required making edits to cladecompare.py and core.py to allow for the hypg to be selected.  

I also added a -nw flag that provides the option of not using the sequence weights.  The hypg module allows for sequence weights to be calculated in an identical manner to the urn module if the -nw flag is not used.