Pluggable storage backends for cb_server, including file-based.
STORAGE_ENGINE environment variable can now be set to "dummy", "file" or "redis". Default is "file".
In "dummy" mode, user records are stored in memory and not persisted.
In "file" mode, a .json file is written for each user into the lib/cb/recs directory.
In "redis" mode, user records are persisted in Redis.
Merge pull request #15 from sourcedelica/ggrossman/pluggable_store

Pluggable storage backends for cb_server, including file-based.