Adding Ability to Get Document Terms, Instantiate Document Object

Barring two issues, we should be able to get a document’s terms from a
Corpus object and display them, and also instantiate a Document object.

DocumentTerms class and model added that functions similarly to
CorpusTerms. (A DocumentTerms object is returned when
Document.getTerms() is called.

Issues Outstanding for Testing:

1) Ext.bind does not seem to be working for the new
Corpus.getDocuments() function when placing the function name in the
transferable list.
2) Document object instantiation does not seem to be working despite
placing a specialized constructor in Document.js similar to
https://github.com/sgsinclair/Voyant/blob/master/src/main/webapp/resourc
es/app/data/Corpus.js#L71