make the plugin able to download behind proxy on windows guest

Currently the plugin is not able behind proxy on windows guest.
I rewrote the PowerShell script to try to download without proxy. If the attempt fails, a new attempt is made using the environment variables http_proxy and no_proxy. The new script also works if the proxy requires authentication.

This also fixes the issue #80

The proxy configuration can be done using  then plugin [vagrant-proxyconf](https://github.com/tmatilai/vagrant-proxyconf).