Seal package exports/namespaces about data-describe

[brianray]

@terrytangyuan any good examples of this one?

[terrytangyuan]

This repo I worked on has been migrated to tensorflow/tensorflow but can be used as an example here: https://github.com/tensorflow/skflow/tree/v0.1

So if you take a look at this preprocessing module, it contains multiple files: categorical.py, categorical_vocabulary.py, text.py. But in preprocessing.__init__.py, we only import methods in preprocessing.text and preprocessing.categorical. This is because preprocessing.categorical_vocabulary is implementation detail and we don't want users to use anything inside that module.

So for this project, for each __init__.py file, we need to be careful and only import the methods that the package maintainer expects the users to use. Leaving __init__.py files empty would result in exposing all the methods to users.