Allow to use `Observer`s as a context manager

This piece of code, taken from the documentation, could be less verbose using a context manager.

Current code:
```python
observer.start()
try:
    while True:
        time.sleep(1)
finally:
    observer.stop()
    observer.join()
```

Proposition:
```python
with observer:
    while True:
        time.sleep(1)
```

Both codes will remain valid, of course.