Simplified/Improve IndexedPiece Tests
Update FrequencyExperimenter and Tests

- FrequencyExperimenter accepts a dict of Series, using each key as the
  column name in the outputted DataFrame.
- FrequencyExperimenter.experimenter_func holds an "ident" value to help
  accomplish same.
- Tests updated for both things.
- Extra empty line removed from experimenter.py
analyzers/__init__.py Imports 'experimenters' and 'indexers'