- Fix a bug in ExclusionsDB; improve URL regexes.
- NLTK's LookupError is actually an IOError.
- Fix bug in __repr__ for CopyvioCheckResult.
- Rewrite YahooBOSSSearchEngine to actually work with oauth2.
- Search engines now take a URL opener in addition to credentials.
* Added exclusions module with a fully implemented ExclusionsDB that can pull
from multiple sources for different sites.
* Moved CopyvioCheckResult to its own module, to be imported by __init__.
* Some other related changes.