Returns a Request object
function download(url,onError) {
    var request = require('request');

    util.log('Downloading cef tarball for '+platform+'-'+arch+'...');

    var requestOpts = {
        uri: url,
        onResponse: true
    }

    // basic support for a proxy server
    var proxyUrl = process.env.http_proxy
                || process.env.HTTP_PROXY
                || process.env.npm_config_proxy
    if (proxyUrl) {
      util.log('using proxy: ' + proxyUrl,'verbose');
      requestOpts.proxy = proxyUrl
    }
    return request(requestOpts).on('error',onError).on('response',function(res){onError(null,res)});
}