Fix python 3 support and add soft display to examples

[hansottowirtz]

I'm mainly using python 3, so I fixed some lines to make it work there (make some strings bytearrays, and a np.arange to range()). I've also added the software display to all the examples, which can be enabled with the MOCK_RPI env variable. I have documented it in README.md.

Also, I added a default gitignore from https://github.com/github/gitignore

Still needs to be verified working with the RPi.