Validate JSON reserved from beanstalk is well-formed

[bradgessler]

When deserializing a job off of beanstalk via Job.deserialize, assert the correct keys exist in the JSON job, args, ttr, ... exist and that no other, invalid keys exist.

If the JSON looks funky, raise an invalid job JSON exception.

Also, consider a better method for Support#constantize that will raise an exception of the object can't be found for better error messaging.