CDN support

The biggest news for this update are that downloading media from CDN's (you'll often encounter this when working with popular channels) now works.

Some bug fixes:

• The method used to download documents crashed because two lines were swapped.
• Determining the right path when downloading any file was very weird, now it's been enhanced.
• The .sign_in() method didn't support integer values for the code! Now it does again.

Some important internal changes are that the old way to deal with RSA public keys now uses a different module instead the old strange hand-crafted version.

Hope the new, super simple README.rst encourages people to use Telethon and make it better with either suggestions, or pull request. Pull requests are super appreciated, but showing some support by leaving a star also feels nice ⭐️