BUG: BUG/TST: Return empty DataFrame on black image, no features, or all features filtered.

BUG/TST: Return empty DataFrame on black image, no features, or all features filtered.
Merge pull request #70 from danielballan/handle-no-features

Handle no features
ENH: Add class for managing framewise data on disk.
API/REF batch uses output class instead of kwargs
DOC: Demo new batch
TST: Update batch tests.
ENH: Add various features to PandasHDFStore.
Merge pull request #69 from danielballan/new-batch

 New batch processing of frames, including new storage API