A couple of major changes:

• documentation of an example now lives in the example folder
• build_docs script assembles the example documentation and ipython notebooks and automatically creates a table of contents in index.md

The motivation:

• make examples and example notebooks easier to contribute: no need to touch docs/ at all
• make example documentation more easily usable: no need to fire up doc server, just look at corresponding readme
• a step toward more complete documentation of the code, automatically assembled from readme's and docstrings

@shelhamer take a look, comment to approve -- i can merge