Move responsibility of resolving URLs to TextContentDocument

Conflicts:
	src/models/book/dtb/segment.coffee
	src/models/book/dtb/textcontentdocument.coffee
Implement Context Viewer with lazy-loading of images

The context viewer is implemented by an iframe whose documentElement is
changed according to context change from segments. Lazy-loading of
images is implemented in hand, because the existing jQuery plugins
didn't work properly when used on an iframe
Remove the stack renderer
Remove espy as dependency
Catch links from the context document
Disable translate3d hack for hardware acceleration

The translate3d hack on iframes seems to chrash iOS devices
Load the book's own stylesheet as well
Use book-dependent ID to check if it's necessary to change context document