MathJax and epubReadingSystem early DOM injection by intercepting WebView requests

[jccr]

Similarly to readium/SDKLauncher-iOS#32
This fixes #32, and #31 and also injects annotations.css

[danielweck]

Awesome @jccr thanks for the quick turnaround! :)

I fixed a bug related to the slash '/' path prefix in URL requests (base directory of the EPUB package sometimes discarded by URL-cleanup utility function): 0b4c0a1

I am now okay to merge to develop.

I successfully tested in both single and two-page layout (synthetic spread = 2x concurrent iframes):

1. MathJax (SVG-based) rendering of MathML
2. annotations.css
3. navigator.epubReadingSystem ("live" object, mutable from each individual iframes context)

Test EPUB for window.navigator.epubReadingSystem:

https://github.com/readium/SDKLauncher-OSX/blob/develop/LauncherOSX/ReaderScripts/epubReadingSystem.epub
(alternative download location: https://github.com/readium/SDKLauncher-iOS/blob/develop/Resources/epubReadingSystem.epub )

(note that this complains about missing ".readium" object, but this is fine as the versioning script is not there yet!)

Test EPUB for MathML:

@rkwright 's SmokeTextFXL (fixed layout, which is great for testing multiple-concurrent iframe injection logic, when two-page synthetic spread is displayed)