New pysquashfsimage command, strings for names and paths, argparse

[AT0myks]

• File names and paths are now instances of str, just like symlinks already were
• Replace fill by the from_bytes class method
• Further code simplifications
• Use argparse for command line parsing
• Add syntax highlighting to the readme
• Add pysquashfsimage command
• Bump version to 0.8.0 (because of added functionality according to semantic versioning)
• Add readme to setup.py so that it shows up on PyPI
• Add Python 2 classifier to setup.py

I would also like to mention that half of the code in the main (everything that's in the else) has always been unreachable. len(sys.argv) <= 1 means there's no argument, so no image to open, and sys.argv[1] raises an IndexError anyway.