Download from blob url instead of data url

a[download] no longer works with data url in Chrome. It downloads the zip as "download.zip".

More info here:
https://code.google.com/p/chromium/issues/detail?id=375634#c2

Tests:
http://web-tests.webatu.com/extensionizr-data/ (old)
http://web-tests.webatu.com/extensionizr-blob/ (new)