Use faster json if available

Use faster json if available

Considered faster according to:
http://stackoverflow.com/questions/712791/json-and-simplejson-module-differences-in-python
http://stackoverflow.com/questions/706101/python-json-decoding-performance

As a side-effect, it might make irker work under Python versions older than 2.6.

Signed-off-by: Eric S. Raymond <esr@thyrsus.com>