Initial context-viewer work

This implements a rudimentary context-viewer - that is, a viewer that takes the content document (often `content.html`) and loads it into an iframe. This allows the user to see all the content in the book - referred to or not - and will (hopefully) make it much simpler to navigate the book.

There's still work to be done, for example removing all the traces of the stack renderer. There's also a lot of decisions to be taken about what we want to include and what we want to blacklist when loading the document.

I've implemented a image lazy-loader, that works quite well (although for now I've just re-used the black spinner icon as loading icon, which is absolutely horrific).

Known bugs:

  - On iOS units, when the user scrolls through the context document, the player controls are also scrolled away. This is due to the crazy way iOS handles iframes. This can be circumvented by applying `position: fixed` to the controls, but I'll have to experiment with this.

"Working" example available at: http://test.m.e17.dk/context_viewer_01/

Feedback is most welcome!