Use json instead of simplejson on python 2.6+

It's a feature request. It's a common practice to use bundled version of simplejson (python's 2.6+ json module) in case the app is running with python 2.6+ and implicitly require simplejson if python is <=2.5 . For example as this done in gpodder multimedia application:

 31 try:
 32 # For Python < 2.6, we use the "simplejson" add-on module
 33 # XXX: Mark as dependency
 34 import simplejson as json
 35 except ImportError:
 36 # Python 2.6 already ships with a nice "json" module
 37 import json

In the code they just use json methods and it works both with native python json module and external simplejson too. The same may be done for json import plugin and twitter/rtm backends.