Add DataFrame interactive widget, widget sub-module, resources, and demo notebook.

[PokkeFe]

• Added the DataFrame interactive widget under the text_extensions_for_pandas.jupyter.widget module header.
• Added necessary resource files to the text_extensions_for_pandas.resources module.
• Added a demo notebook to the notebooks/ path. Note this demo relies on ipywidgets, which does not have a static counterpart and is not completely viewable from a static notebook viewer like Github's.

[review-notebook-app]

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[frreiss]

There are some issues with Python 3.6 and the structure of the import statements. Stack trace:

[...]
~/pd/tep-viz/text_extensions_for_pandas/jupyter/widget/widget.py in <module>
     25 import ipywidgets as ipw
     26 from IPython.display import display, clear_output, HTML
---> 27 import text_extensions_for_pandas.jupyter.widget.span as tep_span
     28 import text_extensions_for_pandas.jupyter.widget.table as tep_table
     29 

AttributeError: module 'text_extensions_for_pandas' has no attribute 'jupyter'

To replicate, use env.sh to create a conda environment with Python 3.6.

[frreiss]

Looking good overall, some minor comments inline.