Moves near-duplicate detection functionality to NearDuplicatesIndex clas...

This commit:
- Refactors Detector class, moving near-duplicate detection functionality to NearDuplicatesIndex class. This allows to separate document representation from its location (until now, documents could only be loaded from the filesystem).
- Tests NearDuplicatesIndex functionality.
- Removes Detector's test, since it wasn't accomplishing its objective.

Motivation behind these changes:
- I had the requirement to calculate near duplicates among html documents, building the index on the fly, and without being able to store the documents in the filesystem.