Callback on Circuit State Change

[lkumarjain]

I have one of the requirement to execute some functionality like reconnect with back-end system as soon as Circuit Open.
Tried to look at code to identify if there is any such functionality is available, but unfortunately there is nothing like Callback on State Change.

Do you have any plan introduce callback on state Change, other than the circuit state change logs.

I will be happy to raise a PR by adding Callback functionility

[lkumarjain]

Create a PR for this #101