Fix #279 once and for all (ha! ;)

This implements the proposal discussed in the re-opened #279 to add a
pin_factory argument at the device level and remove the ability to
specify a pin instance to device constructors (they now only accept a
pin specification).

Note: there's still a couple of bits to tidy up (tests on "real" Pis,
and pin_factory.release_all needs refinement) but the test suite is now
at least capable of passing on a PC.

gpiozero/pins/rpigpio.py

@@ -85,7 +85,7 @@ class RPiGPIOPin(LocalPiPin):
 
     def __init__(self, factory, number):
         super(RPiGPIOPin, self).__init__(factory, number)
-        self._pull = 'up' if factory.pi_info.pulled_up(self.address[-1]) else 'floating'
+        self._pull = 'up' if self.factory.pi_info.pulled_up(self.address[-1]) else 'floating'
         self._pwm = None
         self._frequency = None
         self._duty_cycle = None