Move exceptions to their own sub-module

This removes the circular dependency introduced in PR#137. This also
fixes up an issue in the base meta-class which meant it wasn't working
in Python 3 (only Python 2), and adds a bit to the meta-class to allow
docstrings to be inherited (taken from the rest-docs branch).

gpiozero/exc.py

@@ -0,0 +1,19 @@
+from __future__ import (
+    unicode_literals,
+    print_function,
+    absolute_import,
+    division,
+)
+
+class GPIODeviceError(Exception):
+    pass
+
+class GPIODeviceClosed(GPIODeviceError):
+    pass
+
+class InputDeviceError(GPIODeviceError):
+    pass
+
+class OutputDeviceError(GPIODeviceError):
+    pass
+