Multi book

Multi-book support added. New 'end_book' optional parameter for Passage initialisation function.
New PassageDelta class for extending passages by arbitrary number of verses or chapters
Added Passage.text and PassageCollection.text functions
Deprecated get_passage_text function
Deprecated Passage.extend function
Renamed Passage.osisRef function to Passage.osis_reference
Removed book_total_verses function from Passage object; creating a stand-alone version in its place