Deprecate soco.SoCo.get_speakers_ip() in favour of soco.discover()

I was looking into the code for 'partymode' when I found the call to get_speakers_ip, this appears to be the only place it is used, and there's an old unit-test written for it that is currently disabled, perhaps it's time to deprecate it in favour of the new discover function!

Now there are two possible problems with this change:
 #1: get_speakers_ip runs a lot faster than discover() ( on my network ~ 200ms vs ~ 1200ms )
 #2: get_speakers_ip also returns BR100

I would still argue that it makes sense to deprecate get_speakers_ip and look into optimizing discover() ( if actually needed ).

BR100 can not be used to initialize SoCo, which implies that the one call to get_speakers_ip in partymode shouldn't really return BR100 anyway. ( And now I just realized why I've always seen a error when running partymode )

One way of getting the ips is DRYer than two.

I suggest looking into a separate way of finding BR100, but right now I don't see any usecase where it's needed.