Resource cache invalidation.

This commit secures proper revocation of object URLs created for the
publication fetcher's resource cache.

It also introduces a change to retain original Blob references in the cache.

The rationale is to prevent IE from invalidating object URLs on garbage
collection.
Merge remote-tracking branch 'origin/develop' into feature/resource-cache-invalidation
Merge remote-tracking branch 'origin/develop' into feature/resource-cache-invalidation
Implementation of LRU automatic cache eviction policy.
Merge pull request #69 from readium/feature/resource-cache-invalidation

Blob URL revoke and cache invalidation strategy (management of publication resources during programmatic fetching: cloud reader only, zipped EPUB loading, or exploded EPUB with obfuscated fonts).