Add get_current_media_info().

Hi. Here's a patch that adds a GetMediaInfo call to SoCo. Most of the fields were NOT_IMPLEMENTED or <tag/> in my trivial testing, and I really don't know much about UPNP, but it works!

I use it for a little script like this, to avoid rewinding to beginning of queue unnecessarily:

```python
uri = sonos.get_current_media_info()['current_uri']
if not uri.startswith('x-rincon-queue:'):
    # BUG: rewinds to start of queue item #0
    sonos.play_from_queue(0)
else:
    # already playing queue so we should be good

    # start playback to be sure
    sonos.play()
```

I wish there was a way for Sonos to switch queue -> line in -> queue while preserving the position.