Review comments

@ellisonbg here are the changes:

- Use the standard ipython logo for the Rich Output image example.
- The python.svg output example wasn't working and I wasn't able to quickly figure out why, so I removed it.
- Fixed the d3 example.
- Replace the Express Dictate example with a scrub jay's bird call (similar to the bird calls used in the bird identification machine learning class I took at SciPy).
- Split the Widgets.ipynb notebook into 3: 1) Interact.ipynb, 2) one on the SF example and 3) one on widgets.  
- Fixed the Basic styling trait names that were out of date.
- Added an interact exercise for a function that prints the sum of two numbers.
- Added an interact exercise for a function that prints a string and the length of the string.
- Added a widget exercise that has them display a Text widget and change it's background and foreground colors.
- Added a widget exercise that has them display a widget inside the Popup widget.
- Removed images dir.

I included the custom display logic just so I could refer the students to the list of "special" method names IPython supports by default.  I think I'm going to leave the extra widget styling content, as a back-up plan if I run out of content too early, but I won't cover it if things go as expected (especially the flexible box model stuff).