v0.7.0

Added

• Implemented __getitem__ for the layout enabling for example: app[1, 0:2] = widget.
• Added "on_relayout" event for Plotly widgets.
• Serialize Pandas series objects and Pandas datetime objects.

Breaking

• row_end and column_end are now exclusive instead of inclusive.
• Simplified the add function in favor of the new getitem functionality,
  which is easier to use and familiar to Python programmers.