Key Changes:
Created an interface, IReifiedTextual, which describes the common properties of the Document, Document.Page, Paragraph, and Sentence classes.
This will allow binding and weighting operations to be implemented more easily without having to accept possibly incomplete fragments of text.

The Document, Paragraph, and Sentence classes themselves have been moved from LASI.Core.DocumentStructures to LASI.Core for better discoverabliltiy.

Cleanedup some directories.