Refactor HTTP Error status in a module
Use new Error module
New RateLimit package for API
Return X-Rate-Limit headers
Improvements for the tests on RateLimit
Run redis-server as a sentinel node
Sentinel demo conf
Daemonize sentinel
Use PYBOSSA_SETTINGS env to config RateLimit
Update the test settings
Config sentinel when initializing the app
Return by default error status code 500
Use pybossa.core sentinel node
Reuse the connection pool.
Create redis_master to reuse its connection pool
Unit Tests for RateLimit
Unit Tests for RateLimit

Only testing app, as the RateLimit check is always run before the API call
itself (it is a decorator). Thus, it applies to App, Category, Task and TaskRuns.
Add RateLimit to API calls
Use the RedisMaster node
Refactor RateLimit tests to its own Class