Circuit Panels

* Adds a CircuitBreaker interface
* Adds a Circuit Panel (a map of string names to a CircuitBreaker)

We could take this a bit further:

* [ ] `BreakerTripped`/`BreakerReset` callbacks on a Panel that pass the circuit breaker name.
* [ ] Rename the package to `circuit`.  `circuit.Panel`, `circuit.Breaker`, `circuit.NewTimeoutBreaker()`

Note: includes #9 
If `OnTrip` and `OnReset` are provided for setting the callbacks, then we probably don't need to export `BreakerTripped` and `BreakerReset`.