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/mock.py

@@ -16,6 +16,7 @@ except ImportError:
     from ..compat import isclose
 
 from . import Pin
+from .data import pi_info
 from ..exc import PinSetInput, PinPWMUnsupported, PinFixedPull
 
 
@@ -32,6 +33,10 @@ class MockPin(Pin):
     def clear_pins(cls):
         cls._PINS.clear()
 
+    @classmethod
+    def pi_info(cls):
+        return pi_info('a21041') # Pretend we're a Pi 2B
+
     def __new__(cls, number):
         if not (0 <= number < 54):
             raise ValueError('invalid pin %d specified (must be 0..53)' % number)