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.
Merge pull request #31 from lewagon/04-04-05-use-local-api

Use a local API
Merge branch 'master' of github.com:lewagon/promo-3-challenges
first shot on Rails challenges
second challenges for first Rails day
small rewordings
simplifying challenge
first shot on Rails scaffold challenge
small rewording
small rewording
small rewording
small rewording
small rewording
small rewording
small rewording
two-models challenge
correct typos
correct typos
adding tips for using Bootstrap with rails helpers
adding tips for using Bootstrap with rails helpers