Added application/json support

Somewhat related to https://github.com/felixge/node-formidable/pull/126

Running: `node examples/json.js`

Should give you the following output:

    listening on http://localhost:13532/
    numbers [ 1, 2, 3, 4, 5 ]
    nested { key: 'value' }
    -> post done
    
    Server responded with:
    Status: 200
    received fields:
    
     { numbers: [ 1, 2, 3, 4, 5 ], nested: { key: 'value' } }