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
Tune breakers to close again after opening

Introduces a config in order to parameterize the breaker for tests and
eventually for other uses.

Removes package level now/after mockable functions.

Uses container/ring instead of a hand-rolled array backed ring buffer.

Ups the default minimum observations in order to produce a higher
likelyhood of gathering enough observations to make failureRatio
meaningful.