Depend on simplejson for Python < 2.7

This adds a dependency on simplejson for Python < 2.7 and moves the
stanza to import the appropriate json module into pyleus.compat.

Closes #46
Did you do that test on 2.7? My understanding was that the real performance benefit was on 2.6, but that come 2.7 it was negligible since the relevant improvements had been mainlined.