Automatic ImageMagick install on Windows

This downloads and runs the IM installer automatically.
Shows installer authorization prompt in most security
environments, but otherwise runs unattended. Installs
to usual BinDeps binary path, and does not touch system
PATH or create icons.
Use innounp to extract ImageMagick DLLs

From http://innounp.sourceforge.net/
Because the inno-based installer cannot be run without admin rights.
Merge pull request #165 from ihnorton/autowinbin

Automatic ImageMagick install on Windows