[API] Heterograph (dmlc#657)

* [Feature][RFC] Heterogeneous graph interface (dmlc#553)

* heterogeneous graph interface

* lint

* disable lints

* disable lint checks

* change node_types to dict

* update

* update

* heterograph view

* message passing with types

* clarifications

* graph queries

* clarifications

* moving add_XXX to Base

* from_networkx

* register functions

* Update heterograph.py

* lint.