Make state store configurable
Implement MemoryStore backed by Hash
Move state to state_store
last_trip_time to state_store
Move state logic to StateStore
Remove unused attr
Extract MemoryStateStore
Add specs for State Stores
Add RedisStateStore
Make Memory State Store default
Add registration functionality

To be able to get the state of circuit breakers to for example a status
page, we need to keep track of which circuit breakers are being used.

This is now being handled through Stroombreker.register, which is used
instead of creating Circuit breakers “by hand”.
Add info about StateStores