v0.2.0

• Switched Stoplight.data_store and Stoplight.notifiers over to using
  simple accessors.
• Modified Stoplight::DataStore::Redis to accept an instance of Redis.
• Refactored Stoplight::DataStore::Redis to use fewer keys.
• Created Stoplight::Notifier and subclasses.
• Sent notifications when moving from green to red.
• Renamed Stoplight::Light::DEFAULT_THRESHOLD to
  Stoplight::DEFAULT_THRESHOLD.
• Renamed Stoplight::Error::NoFallback to Stoplight::Error::RedLight.
• Created Stoplight::Mixin#stoplight for easily creating and running simple
  stoplights.