Add get_node_storage and get_edge_storage to CuGraphStorage (#2381)

This PR enables rapidsai/dgl#14  by adding the following functions/classes to CuGraphStore:

1. add_node_data
2. add_edge_data
3. get_node_storage
4. get_edge_storage
5. CuFeatureStorage

Authors:
  - Vibhu Jawa (https://github.com/VibhuJawa)

Approvers:
  - Xiaoyun Wang (https://github.com/wangxiaoyunNV)
  - Brad Rees (https://github.com/BradReesWork)

URL: #2381