v0.5.0

• Data stores and notifiers can be configured on a per-stoplight basis. This
  allows stoplights to use stoplights internally.
• Stoplights use stoplights internally to wrap calls to data stores and
  notifiers. This means they gracefully handle either going down.
• Data stores only store failures and states. Also failures are stored in a ring
  buffer. This drastically reduces the amount of data stored.
• Stoplights will use the fallback (if it's given) when they fail while they're
  green. This means they won't re-raise exceptions if you provide a fallback.
• Stoplights pass the error to their notifiers when transitioning from green to
  red.