[Umbrella] Import\export between Zeppelin and Jupyter notebook formats

As a user doing data analytics in Notebook environment, I would like not to be locked up with a single notebook implementation.

Right now, Apache Zeppelin has it's own notebook format note.json, and it is different from a Jupyther one, so user is locked up.

In order to avoid it, Zeppelin user should be able to:

• import new Jupyter\IPython {.ipnb}

  notebook to Zeppelin, so it gets converted to a

  {note.json}

  and saved under ./notebooks/

• export existing Zeppelin notebook as Jupyter format, so it gets converted and downloaded as Jupyter .ipnb file

This can be further broken down to sub-tasks (in order of priority):

• backend: implement an .ipnb <-> note.json convertor, \w UnitTests
• backend: implement REST API for import\export notebooks in Zeppelin
• frontend: add GUI for Import/Export to Zeppelin webapp

As soon as backend is ready, it will be easy for find people working on frontend, willing to contribute Import/Export GUI.