Use 'json' module if it's available.
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+)

Committing on behalf of David.

Reviewed http://reviews.review-board.org/r/887/




davidt/rbtools,Khan/rbtools,1tush/rbtools,clach04/rbtools,haosdent/rbtools,datjwu/rbtools,halvorlu/rbtools,haosdent/rbtools,reviewboard/rbtools,reviewboard/rbtools,davidt/rbtools,beol/rbtools,blair/pkg-rbtools,halvorlu/rbtools,beol/rbtools,beol/rbtools,reviewboard/rbtools,davidt/rbtools,clach04/old_rbtools_0.3.2,haosdent/rbtools,halvorlu/rbtools,datjwu/rbtools,bcelary/rbtools,clach04/rbtools_0.4.1,Khan/rbtools,datjwu/rbtools