Index for notebook documentation

This is a proposition to:

* add a toplevel Index notebook to guide the reader (similar to IPython documentation http://nbviewer.ipython.org/github/ipython/ipython/blob/master/examples/Index.ipynb)
* add a reference in the Sphinx documentation to the Notebooks (through nbviewer.ipython.org for the html rendering)

What do other people think ?
The notebooks in examples/notebooks are starting points that I convert into examples for cvxpy.org. They weren't intended to be browsed. A lot of them are out of sync with what's on cvxpy.org.