Make CircuitBreaker public

- The constructor still panics on invalid failureThresholds.
- Made breakerHandler private, as golint was complaining about comments,
  and it didn't seem to be usable outside of DefaultHandler, anyway.
- Fixed a Printf-args-mismatch in metrics_test.go

Please review and adjust method comments as desired.
Merge pull request #7 from peterbourgon/make-circuit-breaker-public

Make CircuitBreaker public