Improve Perf of Pure JSON Data

This minor release drastically improves the perf of serializing pure JSON data with the new {isJSON: true} flag. #17

Passing a replacer function to JSON.stringify() slows it down dramatically. If possible, we should avoid this this path, and the new isJSON option is the signal that the object passed-in contains no functions or regexp values. But still protect against XSS by properly escaping for a <script> context.

Node v0.12.10
simpleObj:
JSON.stringify( simpleObj ) x 1,303,349 ops/sec ±0.66% (99 runs sampled)
JSON.stringify( simpleObj ) with replacer x 386,634 ops/sec ±0.72% (96 runs sampled)
serialize( simpleObj ) x 284,535 ops/sec ±0.92% (98 runs sampled)
serialize( simpleObj, {isJSON: true} ) x 951,798 ops/sec ±0.92% (96 runs sampled)