Pytests for core

Quite a few tests for core, which do not need a network connection. May be a few more to come, but could be committed already.
Relies on a typo, which is fixed in #185 

do something like 
```Python
from __future__ import print_function
from soco import SoCo, services
sonos = SoCo('10.0.4.4')
instance = services.DeviceProperties(sonos)
for action, in_args, out_args in instance.iter_actions():
        print("%s.%s " % (instance.__class__.__name__, action))
```
to verify the actual command.