Introducing circuit breaker middleware

Implements circuit breaker similar to  https://github.com/Netflix/Hystrix/wiki/How-it-Works

Vulcand circuit breaker watches the error condtion, in case if the condition matches the realtime metrics,
it activates the fallback scenario, e.g. returns the predefined response or redirects the request to another location.
Merge changes introduced after code review
Merge pull request #114 from mailgun/alexander/circuitbreaker

Introducing circuit breaker middleware