Using properties instead of methods

See #58.  Replacing a number of methods with properties, e.g.:

Use 
```python
soco.volume = 30
soco.volume -=3
soco.status_light = True
```

instead of
```python
soco.volume(30)
soco.volume(soco.volume()-3)
soco.status_light("On")
```

Also updates the Integration tests accordingly.

#45 by @petteraas will need a few changes to take account of this if this is accepted.