Add carbon-relay support

Commits related to `carbon-relay` from issue #44:
* Added **carbon-replay** support (3dbf75b, 0cc44f4).
 * init files and runit scripts templates merged (9da8167, b030286).
* **carbon-cache** recipe **separated** from the main carbon recipe (ea27d1a, ec8d228).
* *local_settings.py* supports setting **cluster_servers** and **carbonlink_hosts** variables (43d841c).
* Added **relay-rules.conf** support (203360c).
 * Configuration **example** added to the README (5c16892).

Bonus:
* Added **tests** for carbon-relay to test this changes (6f0c911).