Use a local API

This commit adds a local API based on Sinatra that can be started
with `rake api` and that serves static assets.

This has been done to prevent XSS limitations and complex workarounds
for students and provide a local API that on the same domain than their
own code.

The API documentation has been added to a local file named API.md, the
online version (Github/Heroku) is then no longer needed.