#26 Have a JSON datatype for elements

[Jeremy Dorn]

Have an application/json datatype for page elements. When calling CMS::load('page','element') on a JSON element, the result is json_decoded as an associative array before being returned.

Possibly use this instead of CSV since it is much easier to encode/decode and all column information is self contained. For this, add a generate_json_plugin function that's almost identical to generate_csv_plugin.