DOC: cleanup examples folder and webpage

I couldn't make the revision history readable, so I just squashed it. I think it's easier to see what I did by looking at the repo rather than the diffs. 

https://github.com/vincentarelbundock/statsmodels/tree/master/examples

Basically, I re-organized examples in 3 subfolders:

1. examples/notebooks: This is where real work happens for "website-ready" examples
2. examples/python: I replaced the old python scripts with new ones that were generated by issuing:
    - ``ipython nbconvert --to python *``
    - This is done at commit time (not when the website is built)
3. examples/sandbox: Example scripts that don't have an accompanying notebook, and that are mostly useful to developers and adventurous users. 

Advantages:

- Putting half-baked examples inside a "sandbox" folder marks them as such, thereby reducing potential for confusion.
- Using IPython to convert notebooks into scripts allows us to: 
    + Keep both scripts and notebooks in the repo
    + Keep notebooks and scripts in sync at essentially zero maintenance cost
- Looks cleaner