remove inferencer and Inference from __All__, since infer is sufficent, they are implementation detail.

rename inferencer.py to inference.py

* Change build process to generate v2 documentation
* Add directory for v2 api
* Correct sphinx index soft-link

I use pre-commit check all file and report is the passed.
But it is not passed in Travis, so I update __init__.py

remove ITrainer interface since parameters for train is already getting out of sync. Currently there is no benefit of having a interface.

expose build_dict in imikolov dataset, fix bug that len(word_dict) is not bigger than all index in word_dict.

change argument name of data_type.integer_value/integer_value_sequence to ele_range, add documentation.