v0.3.0

• Allowed formatting notifications.
• Added automatic recovery from red lights.
• Added Stoplight::DataStore#clear_stale for clearing stale lights.
• Removed forwarded methods on Stoplight module.
• Moved some methods from Stoplight::DataStore::Base to
  Stoplight::DataStore.
• Renamed Stoplight::DataStore::Base#purge to #clear_stale.
• Renamed Stoplight::DataStore::Base#delete to #clear.
• Prefixed data store getters with get_.
• Added Stoplight::DataStore::Base#sync.
• Changed Stoplight::DataStore::Base#get_failures to return actual failures
  (Stoplight::Failure) instead of strings.