Eliminate code duplication:

* use ResourceFetcher's markup parser from ZipFetcher and PlainFetcher instead of using their own private instances.
* Pull up shared logic from ZipFetcher and PlainFetcher to ResourceFetcher
Deobfuscation support for exploded EPUBs
Immediate loading of external stylesheets.

Renamed method and function arguments to clarify expectations for relative path values.
Removal of redundant code.
Implement support for resolving resource references in embedded STYLE
elements for zipped EPUBs or encrypted resources.
Removed redundant debug logging.
Update submodule ref.
Merge pull request #19 from readium/feature/font-mangling

Feature/font mangling