Fix #354, fix #389

Overhaul the pi_info system:

Pin factories are now capable of generating pi_info themselves (although
currently they all just look up the revision and call pi_info with a
specific one).

PiGPIOPin will now return pi_info for the remote pi which can be
specified by parameter or implicitly by the environment vars.

Overvolted Pis should work properly no matter what (some argument over
whether the revision 7 or 8 chars in this case; both should work). Added
some minor tweaks for the new camera-capable Pi Zero

Finally, added a bunch of tests for pins.data

gpiozero/pins/native.py

@@ -17,7 +17,7 @@ from time import sleep
 from threading import Thread, Event, Lock
 from collections import Counter
 
-from . import Pin, PINS_CLEANUP
+from . import LocalPin, PINS_CLEANUP
 from .data import pi_info
 from ..exc import (
     PinInvalidPull,
@@ -149,7 +149,7 @@ class GPIOFS(object):
                     f.write(str(pin).encode('ascii'))
 
 
-class NativePin(Pin):
+class NativePin(LocalPin):
     """
     Uses a built-in pure Python implementation to interface to the Pi's GPIO
     pins. This is the default pin implementation if no third-party libraries