Python 3 Support

[msiebert]

Hi guys, we're getting started with Nutritionix at our startup, ieatfit, and we're using Python 3. I found that this library supports Python 2. So I forked this repo and did the trivial work to get it to work under Python 3. I imagine you probably have people using the current library, so it's unfeasible to merge my fork in, but if anyone asks, you can point them at https://github.com/msiebert/nutritionix-python-3. I've also got instructions on how to install it through pip on my blog http://msiebert.github.io/blog/nutritionix-python-3/. Thanks for the great work you guys are doing!