Fix class variable that was supposed to be instance variable.

Also updated setup.py to state python 3 and python 3.2 support.

Add six dependency to setup.py
Add metadata information to Volume attributes
Adding processing methods into Volume class. Log:

 - add Volume method for getting a list of token counts across the whole volume

 - removed python 2 specific .iteritems

 - Spin off TermIndex into its own module

 - Add Volume.frequencies() for a term-page frequency (or occurrance) index

Add term_page and term_volume frequency methods for Volume