Lazy caching model not thread-safe

It isn't worth the effort to try and figure out thread safety, so that
approach is being junked.  

New Classes:
- `edu.rit.honors.drive.model.DriveFilesystemContainer` is adapted from
  `com.vaadin.data.util.FilesystemContainer`
- `edu.rit.honors.drive.HelperFactory` allows for backend independance