Change MockPin (and MockPWMPin) to make them behave more like 'real' pins - fixes#206
Add new MockPin tests, and rework some of the existing ones
Incorporate #216
* rename FUNCTION / PULL / EDGES dictionaries so that they better-match the other `Pin` implementations
* throw `PinSetInput` if trying to set the state of an input pin
This commit is a fairly major piece of work that abstracts all pin
operations (function, state, edge detection, PWM, etc.) into a base
"Pin" class which is then used by input/output/composite devices to
perform all required configuration.
The idea is to pave the way for I2C based IO extenders which can present
additional GPIO ports with similar capabilities to the Pi's "native"
GPIO ports. As a bonus it also abstracts away the reliance on the
RPi.GPIO library to allow alternative pin implementations (e.g. using
RPIO to take advantage of DMA based PWM), or even pure Python
implementations.