add newline at end of file
add assert messages.
clarify spec for IndexedPiece.add_index
clarify error message
remove unnecessary text
update Indexer to be more self-consistent

- default values for the class variables should make sense (i.e. if the variable is supposed to be iterable, use something iterable by default)
- init should only scan through the score if it has to
update tests for IndexedPiece.add_index

- mock class to stub the analyzers.indexer module
- a few more cases in the test body