Minor code and repo cleanup

• Remove unused imports
• Provide an actually matching requirements.txt
• Add .gitignore for cache and data files
• Apply style fixes from Flake8 except where noted in provided .flake8 config
• Improve Markdown formatting of README.md
• Use exclude instead of blacklist
• Configure Metadata URL for analyze.py the same way as for main.py (instead of hard-coding some URL and commenting that one needs to change it)