Manual URI item

This implements a (very) minimal first version of a URI data structure that can be passed to ``add_to_queue``. A the moment it is not possible to set the title or provide a MIME-type of any of the other stuff that we might need for this class, quite simply because I could not get it to work. I think that if Sonos does not like the metadata it simply ignores it. I have tried with modifies versions of the music lib track, music service track and radio station and none of it seems to work, so at the moment it is simply an ampty DIDL-Lite element. Any ideas about how to make this metadata look are most welcome.

An example of how to use the class is:

```python
import soco
from soco.data_structures import URI

soc = soco.SoCo('...ip_address...')
uri = URI('http://www.noiseaddicts.com/samples/17.mp3')
soc.add_to_queue(uri)
```

Note. This PR is based of #140, which is not commited yet. So only the last commit is actually associated with this PR.