Fix handling of simplejson vs json packages

The "json" package became available in Python in 2.6, and we
were blindly trying to import "simplejson", but left it out of the
requirements.

Require simplejson if running < 2.6.  Otherwise, try to import
simplejson (it has speed optimizations) and if not found, fall back
to the standard "json" package.
PEP8 and Pylint fixes