Allowing jobs to be put on the queue by string in addition to klass.

This is the Ruby client equivalent to
https://github.com/seomoz/qless-py/pull/2

The rationale behind this is avoid forcing the job module to be importable from the caller code. 
This allows job code to only reside at the worker, it would ease up eventual import headaches and it would in principle allow for putting jobs on a queue from a language (e.g. Python) and have workers implemented with another language (e.g. Ruby) pick up the job and run.

I have modified the code and written corresponding tests.

I'm no Ruby expert so please evaluate with care.