Make windows installers quiet,  issue #572

The possibility for a silent numpy and/or scipy installer has been requested a number of times.
I have made a number of changes that should enable this, please review this. I'm not familiar with compiling numpy, so this is untested!

The basic idea is to create MSI installers instead of exe's. Then we can use msiexec and pass it a silent flag if the NSIS installer was passed a silent flag. This can be achieved using the NSIS 'IfSilent' directive.