Allow job creation with func as string references.

As rq-scheduler just passes the Job creation off to the rq library and
it accepts functions as string references, we remove the check for
`__main__`.

See Issue #54.

I'd also be happy changing this PR to be something more like `if hasattr(func, '__module__') and if func.__module__....` instead of removing the check altogether.  Whichever you'd prefer!  Thanks again for your consideration!