Cache built using redis-py

This is a decorator that caches objects into Redis, using a master-slave
approach thanks to the sentinel feature of Redis.

Sentinel handles automatically client discovery, and creates master/slave nodes
for a high-available and load balance cache.
Add cache decorator

This commit allows to cache using a key_prefix. It also allows to read the
config parameters from the settings file.
Use new customized cache system
Change to new Cache
Use new cache system
Add delete_memoized and delete_cached functions to clean cache
Use new cache system
Use new cache
Hash the arguments
Use new cache system
Use new Cache system
Enable via config file the cache
Add an option to disable cache and do the tests
Disable CACHE when testing in Travis
Disable cache with ENV variable
Use upstream redis-py
Add documentation about installing Redis as the new cache system
Disable CACHE if settings_local is not found (for Travis)
Remove unnecessary import
Fix setting os environ