Adding library JSONX to benchmark.  JSONX very fast.

````
encode:            json:           198792
encode:           jsonx:           106736
encode:      ejson_test:          1056210
encode:      mochijson2:          1004936
encode:           jiffy:           496371

decode:            json:           294628
decode:           jsonx:           147427
decode:      ejson_test:          1258747
decode:      mochijson2:          3537416
decode:           jiffy:           332761
````