Add support for cmdenv verification before launching job

[bwhite]

A common source of problems is that a job needs a certain cmdenv (a way of passing parameters to jobs as environment variables like os.environ['my_param']) and the user will forget to add this. A simple fix is to add a list of required cmdenvs to the hadoopy.run command so that the user can be notified that a required variable is not present. A more sophisticated solution would be to allow an arbitrary function to validate the arguments (e.g., is it an int, etc.) but this may be too complex.