fixing doc inconsistency, bad variable name 'id', and None comparison

This PR does the following changes:

Docstring mentioned process_img_batch instead of process_image_batch.
Reserved word id was used as a variable name, replaced it with id_ as PEP8 recommends (class, id, map, filter, hash_, ...).
Changed None comparison from if variable == None to if variable is None to avoid numpy elementwise object comparison in the future.

heuritech / convnets-keras