Cleanups

[akx]

Some small cleanups.

• __init.py__ should be __init__.py.
• No other file needs to be executable than print.py.
• requirements.txt contained a bunch of dependencies that aren't required by the app.
• Error messages were being both printed and raised; just catch the RuntimeErrors instead
• Previewing doesn't really belong in read_img, so refactor it out
• Move the catprinter logger to the module, so you don't need to pass it around
• Fix up some flake8 complaints (but not quite all of them yet)

[rbaron]

Thanks @akx, looks great.