Rewrite indexer.py Indexing Algorithm and Tests

- Indexer handles Stream and Series objects differently
  - Stream uses previous algorithm
  - Series uses pandas techniques
- No need for ElementWrapper objects any more
Simplified vis.analyzers.indexer._series_indexer()

Use pandas.DataFrame.apply() rather than a python loop.
Merge branch 'visX-devel' of github.com:crantila/vis into visX-devel