Add support for JSON.parse reviver

Similar to [object_hook][object_hook] in python, ES5 JSON.parse accepts
a very not well-known [reviver][reviver] argument.  This can be used to
easily plug in to existing middleware like [body-parser][body-parser]
for express:
 ```javascript
 var EJSON = require('mongodb-extended-json'),
  bodyParser = require('body-parser'),
  app = require('express')();
app.post('/save', bodyParser.json({reviver: EJSON.reviver}), function(req, res){
  console.log('omg now it has extended types', req.body);
  // Do stuff
});
 ```
[object_hook]: https://docs.python.org/2/library/json.html#json.load
[reviver]: http://mdn.io/json.parse#Example.3A_Using_the_reviver_parameter
[body-parser]: http://npm.im/body-parser