Highly improved Chrome extension

Full list feature changes in this commit:
- Support for iframes
- Switched to content-type (MIME) detection instead of hard-coding a
  case-sensitive check for the .PDF extension
- The PDF's original URL is visible in the omnibox
- Support for incognito mode

Note: PDF viewer is disabled for the file:// + incognito
combination, because it's currently impossible to get the combination
to work.

See https://github.com/mozilla/pdf.js/pull/3017#issuecomment-15693432
Compatibility with XHTML documents

Previously, when the XHTML doctype + header is active, checks
would fail because a <div>'s tag name is "div" instead of "DIV".

document.activeElement does not exist in Chrome for XHTML documents

== -> ===
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
Merge pull request #3017 from Rob--W/chrome-extension

Chrome extension - Highly improved