Fixed download.py (Empty file downloads, encoding errors)

Fixed the following errors -
1. Python defaulting to ASCII encoding instead of Unicode.
Set global encoding to 'utf-8'
2. Sript downloading 0kb empty FILE objects.
Some files being generated contained ":", which is a restricted
character in filenames on Windows and MacOS and was truncating filenames
before the extension, added to the replacements list in clean_text.
3.  Download failures.
Used the requests library to improve success rates of downloads, which
seemed to work a bit better.