Ignore specified labels & other features

- partial evalb prm file support:
  - parameter DELETE_LABEL is supported so that
    root & punctuation can be ignored.
- support lemma field of export format
- use (multi)set operations
- add exact match
- unlabeled evaluation as separate option (avoids duplicate code)
- read everything as UTF-8 unless otherwise specified
- require python 2.7+ and support python 3.3+