Add Support for Importing Opus Objects (WIP)

- implement "known_opus" Parameter in IndexedPiece.get_data()
- make OpusWarning class
- add appropriate tests
Add Support for Importing Opus Objects (WIP-2)
Add Support for Importing Opus Objects (WIP-3)

Now IndexedPiece._import_score() does what it needs. With tests too!
Add Support for Importing Opus Objects (Finished; Cleaning Up)

Closes issue #234
Merge branch 'master' of github.com:ELVIS-Project/vis