Asset purge: Removes infringing assets and binaries + downloader script

[Sebelino]

I suggest storing the copyrighted assets and other binary files elsewhere. Nintendo isn't reluctant to take down GitHub repositories, and tracking huge binaries should generally be avoided anyway in my opinion.

I untracked any hint of copyrighted assets (PNGs, MP3s, etc.) and moved them to a separate file server. I then wrote a Python2 script that downloads those assets and places them in their appropriate directories. Here is how to download the assets to a download directory (./assets/) and then "unpack" them to the root directory:

$ python2 manage_assets.py download
$ python2 manage_assets.py unpack

Or just:

$ make getassets

There is also support for checking file integrity against assets.md5; see the bottom of manage_assets.py for details. The list of asset paths and mirror URLs are hard-coded into the script.