Chromium extension using streamsPrivate API!

Opera 19 has finally added support for the streamsPrivate API, and whitelisted the [PDF Viewer hosted on addons.opera.com](https://addons.opera.com/extensions/details/pdf-viewer/) with (extension ID encfpfilknmenlmjemepncnlbbjlabkc).

This feature was developed 5 months ago at https://github.com/Rob--W/pdf.js/issues/1. The main new features are support for POST requests and FTP urls.

See commit messages for implementation notes.
To test the extension in Opera 19, follow the following steps:

1. Get Opera 19 from http://www.opera.com/computer/next (I've tested with v19.0.1326.26).
2. Build the extension (instructions at https://github.com/Rob--W/pdf.js/issues/1), or get a test build from https://robwu.nl/pdfjs/pdfjs-0.8.877-streamsPrivate.zip
3. Unpack the zip file.
4. Start Opera, and go to Extensions (Ctrl + E).
5. Click on the button to enable Developer mode.
6. Click on "Load unpacked extension".
7. Use the file browser to select the directory containing the contents of the extracted zip file (or if you've built yourself: `pdf.js/build/chromium`).
8. To verify that the new feature works as intended, try to view a PDF file from a FTP site. Just Google for "ftp pdf" and pick any search result that serves a PDF file over `ftp:`.

Unfortunately, Chromium has not yet responded to the request of getting the extension ID whitelisted: https://code.google.com/p/chromium/issues/detail?id=326949 The extension will gracefully fall back to the current behavior in Chrome/Chromium.

Fixes issues:
- https://github.com/operasoftware/pdf.js/issues/8

/cc @andreasbovens