Improve sort_files utility to remove empty folders.
Monster commit. Add features and re-factor code.

* Add imgur.py, which parses an Imgur album URL for images.
* Add android.txt and mobile.py to allow for Android compatibility.
* Refactor main.py into download.py to allow for external calls, rather than only CLI
* Refactor download.py to remove unnecessary duplication.
* Move some common functionality into utils.py