Completely overhauled file system

// code changes

- Instead of opening each cache-file, an index is kept.
- If the max. age of a cache-file is reached, the script won't create a new file anymore - it overrides the old one.
- Errors are now reported through exit(), the script will stop executing immediately.

// general changes

- The license-link has been changed, it's no longer depending on Google's infrastructure (GitHub is used).
- The slogan was changed into a more general form.