Use 'json' module if it's available.

Review Request #887 — Created June 7, 2009 and submitted

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Use 'json' module if it's available.

simplejson is now part of the Python standard library, but it's named 'json'.
This change alters post-review to use it if it's available, which reduces the
amount of stuff people have to install (if they have Python 2.6+)