Fixed bug in get_group_coordinator

    This fix addresses issue #64. The get_group_coordinator() method
    returns ‘None’ for a zone which is in Stereo mode, when invoked on a
    device which is a slave.

    The method also needs to be invoked on a device that is initialized
    with an IP address that belongs to the zone. This should not be the
    case.

    I’ve refactored the method to get the topology of all devices and
    extract the coordinator for the given zone name. This allows us to get
    the coordinator IP for any zone from any initialized SoCo object.
Merge branch 'get_group_coordinator' of https://github.com/murali44/SoCo into get_group_coordinator