Skip listing elements with nil value.

Fixes #2
Fix to_ruby conversion on nil values

See #2
Don’t stacktrace for recoverable errors.

 * Skip arguments that aren’t files (with a warning on STDERR)
 * Print warnings for types that don’t match (e.g. `add` or
   `cleanup --sort` on anelement that isn’t a set
 * Abort classification if no files with appropriate elements
   are provided for training

Fixes #1
Bump to v0.1.1