Stop fetching media assets programmatically if the EPUB is exploded.

This commit introduces exception to programmatic fetching for media
assets when dealing with exploded EPUBs.

Programmatic fetching can be currently triggered for exploded EPUBs if
they declare obfuscated fonts in META-INF/encryption.xml.

Since such encryption cannot currently apply to media assets (videos,
audio files, images), it makes no sense to programmatically fetch those.

For this reason, in such a case references to those assets are left
untouched, they aren't fetched programmatically and the base URI of the
programmatically constructed content document is configured so that
those media asset references are resolved correctly by the browser.
Merge pull request #43 from readium/feature/media-fetching

Stop fetching media assets programmatically if the EPUB is exploded.