Issue rate limiting

This pull requests adds a Rate Limiting feature to all API calls, to avoid abuses on the server side.

It follows the same guidelines from [Twitter](https://dev.twitter.com/docs/rate-limiting/1.1) and it is based on the code from [Armin Ronacher](http://flask.pocoo.org/snippets/70/) licensed as public domain. The code has been modified to support the Redis Sentinel mode, so the master node is configured via it.

Additionally, Vagrant, Chef Cookbooks and documentation have been updated to integrate the changes.