Added sleep property. Property puts the OLED display to sleep or wakes it up after sleep. Thanks kalehmann!
To use in CircuitPython, simply install the [Adafruit CircuitPython Bundle](https://circuitpython.org/libraries).
To use in CPython, `pip3 install adafruit-circuitpython-macropad`.
Read the [docs](http://circuitpython.readthedocs.io/projects/macropad/en/latest/) for info on how to use it.