Separate the page action functionality

Until now, the extension URL router and the page action code
lived in the same file.

However, some applications consuming PDF.js might want to use
one of these functionalities, but not both.

So I separated the functionalities into 3 files:
 - extension-urls.js contains the common base
 - extension-router.js sets up the extension URL router
 - extension-pageAction sets up the page action.

All three files are now included in pdfHandler.html.