simplejson is a native code version of the json module in python
(the json module in core python implements the simplejson api), which
means that simplejson can be used in place of the json module, but it
performs much better. This patch attempts to load simplejson, and falls
back to normal json if it fails.

In the summary path on a single run of quick.tests, simplejson is about
3-4 seconds faster than json, ~7 seconds vs ~11 seconds.

Signed-off-by: Dylan Baker <baker.dylan.c@gmail.com>