Added a power property to be able to tell the whether the display is on or off. Thanks iraytrace!
To use in CircuitPython, simply install the [Adafruit CircuitPython bundle](https://circuitpython.org/libraries).
To use in CPython, `pip install adafruit-circuitpython-ssd1306`.
Read the [docs](http://circuitpython.readthedocs.io/projects/ssd1306/en/latest/) for info on how to use it.