Move content document fetching logic from iframe_zip_loader to publication_fetcher.
Extract content document related logic to a new content_document_fetcher submodule.
Extract deobfuscation/decryption logic to a new encryption_handler submodule.

Small naming fixes in publication_fetcher.
Resource fetcher naming consistency fixes.
Elevate the resource cache onto the global EPUB-wide level.
Merge pull request #26 from readium/feature/epub_fetch-refactoring

Feature/epub fetch refactoring