Rename DefaultBreaker to Handler

Per recent discussion, we decided that each functional package would
provide a Handler method (wrapping and returning an http.Handler
suitable for servers) and a Transport method (wrapping and returning a
http.RoundTripper suitable for clients).
gofmt -r 'time.Now().Sub(x) -> time.Since(x)'
breaker: move Handler to separate file

Prepare for Transport.
proxy: a proxy-everything HTTP transport
breaker: rename CircuitBreaker to Circuit, per discussion
breaker: rename breakerHandler to simply handler
breaker: add Transport implementation

Also, re-word some Handler docs and tests for consistency.
Rename Circuit to Breaker; refactor API

- Handler and Transport paramaterize all of their behavior.

- Both take a Breaker, which is now an interface.

- Both also take a validator, to pass/fail responses from their wrapped
  Handler or RoundTripper.

- Privatized breaker.{trip,reset} methods, as they were only being used
  in tests.
Merge pull request #8 from peterbourgon/circuit-breaker-roundtripper

Circuit breaking Transport