date reviver now optional

Example node session:

> jh=require('./index.js');
{ parseStr: [Function],
  dateReviver: [Function] }
> jh.parseStr('{"date":"2013-07-10 CST"}');
{ header: { date: '2013-07-10 CST' },
  body: '' }
> jh.parseStr('{"date":"2013-07-10 CST"}',jh.dateReviver);
{ header: { date: 'Wed Jul 10 2013 16:46:14 GMT-0500 (CDT)' },
  body: '' }
fixed quote problem and added tests

To run the tests, either run "runtests" or "mocha". The tests use mocha and
expect.js.