Improve download button and behaviour

Fix download button for top frames and sub frames

When PDF.js is the top frame, and the PDF URL is identical to
the top URL, download would fail. Fixed by adding a ? or & in these
cases.

When PDF.js is embedded in a frame from a different origin, download
would fail because window.open(url, '_parent') is ignored.
Fixed by using a.click() when available.
a.click() works in Chrome 25, Firefox 19, Opera 12.00 and IE 8.
Safari 5.1 does not support a.click()

Use a.download if available + documentation
Enable Web Worker again in Chrome extension

Previously, Web Workers were not created because of a security
policy which does not respect relaxed privileges.
Added Chrome extension's author