Add support for nonstr keys

I've updated setup.py to be explicit that
we're requiring python2.6 and greater, because
I'm using collections.MutableMapping.
Merge branch 'non-string-keys'

Closes #10 and #11.

* non-string-keys:
  Add support for nonstr keys
Fix ping() bug, should return True not PONG

Verified this passes on the latest version of redis (2.6.10).