Isolate job payload in job data
* Data passed into MongoQueue.put is stored under the key 'payload' in self._data
* Other job info (e.g. locked_by, attempts) is stored in self._data as before
* Job.payload added, returns job._data['payload']
Increment version number.
Updated readme.
Merge pull request #1 from jaycee/master

Isolate job data from job meta data