Added perfdata processing

Hi, 
in the search for a nagios plugin to read status info from a JSON API, I stumbled upon this fork. Made some revisions and added a routine for outputting performance data which can be parsed by Graphite and Carbon.

See https://github.com/phrawzty/check_http_json for a similar plugin in Ruby.

Thought others might have use for it!

Regards,
Benjamin Rokseth 
Oslo Public Library