Added Cambridge Audio discoverable. (#191)

* Added discoverable for Cambridge Audio StreamMagic platform.

* Fixed flake8 complaints.

* Fixed comment refering to different document.

* Removed duplicate attribute assignments.

netdisco/discoverables/cambridgeaudio.py

@@ -0,0 +1,18 @@
+""" Discover Cambridge Audio StreamMagic devices. """
+from . import SSDPDiscoverable
+
+
+class Discoverable(SSDPDiscoverable):
+    """Add support for discovering Cambridge Audio StreamMagic devices."""
+
+    def get_entries(self):
+        """Get all Cambridge Audio MediaRenderer uPnP entries."""
+        return self.find_by_device_description({
+            "manufacturer": "Cambridge Audio",
+            "deviceType": "urn:schemas-upnp-org:device:MediaRenderer:1"
+        })
+
+    def info_from_entry(self, entry):
+        """Get most important info, which is name, model and host."""
+        info = super().info_from_entry(entry)
+        return info