Implemented new params in Endomondo.workout_list(): 'max_results' and 'before'.
'max_results' defines how many entries should be returned (limit for 40 seems to be lifted), and 'before' - either a datetime object or iso formatted datetime string with timezone info:
```python
sports_tracker = Endomondo(email, password)
workouts = sports_tracker.workout_list(10)
[...]
more_workouts = sports_tracker.workout_list(10, workouts[9].start_time)
```

This closes issue #1.