Feature/font mangling

Hi!

This pull request contains the following changes:

* Support for deobfuscation of mangled fonts for exploded EPUBs
* Support for resources and stylesheets referenced from HEAD > STYLE elements in zipped EPUBs
* Elimination of the "Flashing" effect during font loading from zipped EPUBs
* Shared logic elevated from ZIP and plain fetchers to the resource fetcher level

The changes are rebased against the current state of the develop branch and tested using the current version of readium-js-viewer.