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:

* [x] `BreakerTripped`/`BreakerReset` callbacks on a Panel that pass the circuit breaker name.
* [x] Rename the package to `circuit`.  `circuit.Panel`, `circuit.Breaker`, `circuit.NewTimeoutBreaker()`. (defer to future PR)

Note: includes #9 
If the breaker being added to the panel has already set `OnTrip` and `OnReset` callbacks, they're being clobbered here. We need to run both callbacks if both are provided. The callbacks should also be run in goroutines.