Fix #459 - properly support remote SPI with pigpio

Sorry! Dave's messing around with the pin implementations again.
Hopefully the last time. The pin_factory is now really a factory object
which can be asked to produce individual pins or pin-based interfaces
like SPI (which can be supported properly via pigpio).

tests/conftest.py

@@ -0,0 +1,10 @@
+from __future__ import (
+    unicode_literals,
+    print_function,
+    absolute_import,
+    division,
+    )
+str = type('')
+
+import os
+os.environ['GPIOZERO_PIN_FACTORY'] = 'mock'