Change CompositeDevice to reject invalid identifiers

Also updated StatusBoard and StatusZero to reject duplicate identifiers
(namedtuple doesn't pick 'em up because they're passed in a dict and
thus the dups are squashed prior to the call). Added tests for all the
relevant stuff.

gpiozero/devices.py

@@ -330,8 +330,7 @@ class CompositeDevice(Device):
                 raise CompositeDeviceBadDevice("%s doesn't inherit from Device" % dev)
         self._named = frozendict(kwargs)
         self._namedtuple = namedtuple('%sValue' % self.__class__.__name__, chain(
-            (str(i) for i in range(len(args))), self._order),
-            rename=True)
+            ('device_%d' % i for i in range(len(args))), self._order))
 
     def __getattr__(self, name):
         # if _named doesn't exist yet, pretend it's an empty dict