Fix uninstall bug on Windows

[daniviga]

Fixes #29

How to test:

• Download and run http://ftp.openquake.org/windows/testing/OpenQuake_Un_Test.exe
• Install it on a custom location (not the default one!)
• Re-run the installer
• Check if it detects the old installation and if it's able to remove it before prompting for a new installation

Can be tested with Wine too:

export WINEPREFIX=$HOME/winetest
winecfg #change OS to Windows 7
wine OpenQuake_Un_Test.exe
wine OpenQuake_Un_Test.exe #again
rm -Rf $HOME/winetest

[hascar]

LGTM