The sha256sum and md5sum utilities are available by
default on Linux, but not Windows or Mac OS.
I created the script `checksum.py` that can perform
sha256 and md5 checksum verification on all 3 systems.
Got rid of the ugly checksum.bat I was using on Windows.
Deleted the md5 checksum files, since I only need sha256
for now.
Before this change, I was always skipping verification
of downloads on Mac systems.
