Adafruit-circuitpython-si5351

- Added `setup.py`
- Added PyPI release info to `.travis.yml`
- Updated `requirements.txt`
- Added a more comprehensive `.gitignore`

To use in CircuitPython, simply install the [Adafruit CircuitPython bundle](https://github.com/adafruit/Adafruit_CircuitPython_Bundle).

To use in CPython, `pip install adafruit-circuitpython-si5351`.

Read the [docs](http://circuitpython.readthedocs.io/projects/si5351/en/latest/) for info on how to use it.

* Same as 1.0.1 but with fixed release artifact location.

To use in CircuitPython, download the .zip file and copy its contents to the lib folder on the CIRCUITPY drive. Or, simply install the [Adafruit CircuitPython bundle](https://github.com/adafruit/Addafruit_CircuitPython_Bundle).

Read the [docs](https://circuitpython.readthedocs.io/projects/si5351/en/latest/) for info on how to use it.

* Fix `outputs_enabled` property. Thanks to tdicola !

To use in CircuitPython, download the .zip file and copy its contents to the lib folder on the CIRCUITPY drive. Or, simply install the [Adafruit CircuitPython bundle](https://github.com/adafruit/Addafruit_CircuitPython_Bundle).

Read the [docs](https://circuitpython.readthedocs.io/projects/si5351/en/latest/) for info on how to use it.

Initial release for MPY generation.