Circuit Panel implementation

remove the Reset() return value

allow a way to reset a ThresholdBreaker's failure count

doh

CircuitBreaker => TrippableCircuitBreaker

I want to use CircuitBreaker for the interface

add an interface that all CircuitBreaker types implement

add a no-op breaker

add a panel

add a way to get a slice of circuits from a panel

add trip/reset callbacks to a panel

Move noop out of the errors var group, there is no relation

TrippableCircuitBreaker => TrippableBreaker

Don't export callback fields, use only OnTrip() and OnReset()

ResetFailures should be an internal part of a Frequency type breaker

Some doc fixes

Panel callbacks should be run in goroutines

I'm not finding this used anywhere

OnTrip/OnReset callbacks need to chain so Panel's callbacks don't overwrite them

Test panel callbacks

Update some callback docs

Closes #10