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/rpigpio.py

@@ -9,7 +9,7 @@ str = type('')
 import warnings
 from RPi import GPIO
 
-from . import Pin
+from . import LocalPin
 from .data import pi_info
 from ..exc import (
     PinInvalidFunction,
@@ -24,7 +24,7 @@ from ..exc import (
     )
 
 
-class RPiGPIOPin(Pin):
+class RPiGPIOPin(LocalPin):
     """
     Uses the `RPi.GPIO`_ library to interface to the Pi's GPIO pins. This is
     the default pin implementation if the RPi.GPIO library is installed.