Optional job description

Is there a way to add custom description to jobs? Since RQ uses the func_name for its jobs, instance methods from different instances have the same description right now. E.g.:

```python
class Base(object):
    def ping(self):
        pass

class Foo(Base):
    def ping(self):
        # Foo related ping logic
        pass

class Bar(Base):
    def ping(self):
        # Bar related ping logic
        pass
```

If I enqueue both Foo().ping and Bar().ping, I see them as [ping(), ping()] in jobs list and it's hard to tell which one is which.

It would be nice if I could pass a description to a job, like:

```python
q.enqueue_call(Foo().ping, description='Foo.ping() whatever') 
````