batch_mode behavior of tasks and other magics

[BoPeng]

This ticket is related to #247

When we use nbconvert, there is no frontend_comm so some of the notebook features will not be available. In particular, the tasks table, background mode etc will not work.

This ticket is created to implement and test the behavior of tasks and magics in batch mode.

[BoPeng]

The problem here is that the task status tables are created and maintained from the frontend, and the backend just send "updates" to it. If we are to keep them also in batch mode, we will have to use regular display_data and update-display-data, which requires that the backend keeps all the information and send then to frontend using update-display-data, which is not necessarily a bad thing.