Added PDF support - we can now annotate PDF documents

Changes:
 * Update dom-text-mapper to fcd26632 (master branch)
 * Update dom-text-matcher to fcd26632 (master branch)
 * Update Annotator to f1e08bdd (maintenance branch)

 * Updated assets.py for the changed Annotator files and libraries
 * Updated JSChannel with a patch to allow talking to pdf.js
 * Updated our code to follow Annotator API changess
 * Updated the heatmap for two-phase anchoring and virtual anchors

The change in embed.txt fixes a sporadic code insertation problem.

For pages with no script tags, our embed script sometimes crashed.
I have already fixed this problem for YepNope (see #607), but now
it has come back in our own code.

I am not sure why have not this happened before,
but it definitely was happening now.

I applied the same fix that worked with YepNope. It worked again.

   * * * 

This PR obsoletes #890; has been rebased to latest develop, and reacted to all the comments from the code review.