Add mingw-get.zip

This is an exact copy of mingw-get-0.5-mingw32-beta-20120426-1-bin.zip
found on Mingw's Source Forge download page at:
https://sourceforge.net/projects/mingw/files/Installer/mingw-get/mingw-get-0.5-beta-20120426-1/
(You can verify this by downloading both versions and
comparing their respective sha256sums if you like).

The reason I'm copying it here is simply that I haven't found a way
to download from Source Forge in a Windows script using only
pure Windows features (meaning without first having to install
some tool manually).
This strict requirement is necessary for a good bootstrap script
to get interested people into GnuCash development on Windows
with the least amount of hassle.