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
~~~~