Pure Ruby downloader and unzipper

[agross]

Hey @colszowka,

My fork contains some changes around the download and unzipping of the phantomjs binaries. I work mostly on Windows where curl, wget, unzip and the like are not available without further work. I've added code to use pure Ruby to download (with progress) and Ruby's zip instead of unzip. For Linux it stays the same with bunzip2 and tar.

Are you interested? I'd send a pull request.

Thanks,

Alex