PassageDelta class (multi-book not yet supported)
Merge branch 'passage-delta' into multi-book
book_n renamed to start_book_n

Setter and getter created for old variable name
get_passage_text function deprecated in favour of new Passage.text and PassageCollection.text functions
Number of verses rounded up after calculating proportion_of_book*book_total_verses
Unit-test adjustments
is_valid updated for multi-book passages
number_verses updated for multi-book passages
book_total_verses function made stand-alone
proportion_of_book and truncate functions update for multi-book passages
Tests and bug fixes for truncate function
complete_book and complete_chapter functions updated for multi-book passages
Passage reference_string function updated for multi-book passages
osis function renamed and updated for multi-book passages
Unit tests for passage collections reference strings that include multi-book passages
Spaces to line up chapter data in last_verse_data list
PassageCollection.reference_string and check_reference functions updated for multi-book passages
PassageDelta now works with multi-book passages
Misc bug fixes
Flipped PassageDelta binary parameter, to make more sense for users