Adafruit-circuitpython-tsl2561

* Patch example to add delay after enable.

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

To use in CPython, `pip3 install adafruit-circuitpython-tsl2561`.

Read the [docs](http://circuitpython.readthedocs.io/projects/tsl2561/en/latest/) for info on how to use it.

* Add threshold functionality

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

To use in CPython, `pip3 install adafruit-circuitpython-tsl2561`.

Read the [docs](http://circuitpython.readthedocs.io/projects/tsl2561/en/latest/) for info on how to use it.

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

To use in CPython, `pip3 install adafruit-circuitpython-tsl2561`.

Read the [docs](http://circuitpython.readthedocs.io/projects/tsl2561/en/latest/) for info on how to use it.

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

To use in CPython, `pip3 install adafruit-circuitpython-tsl2561`.

Read the [docs](http://circuitpython.readthedocs.io/projects/tsl2561/en/latest/) for info on how to use it.

- 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-tsl2561`.

Read the [docs](http://circuitpython.readthedocs.io/projects/tsl2561/en/latest/) for info on how to use it.

Thanks to dhalbert for finding the issue. Thanks to jerryneedell and caternuson for getting it fixed up!

Resolves:
- Fixes lux calculation to accept ratio of 0
- Simplifies if/elif tests to only test upper limit on each case