Add local reducer and method for change it

[ananas7]

Sometimes we need to get store after use actions. Actually it's useful for testing containers. I found method replaceReducer, but it's not worked.
Method replaceReducer do nothing

replaceReducer (nextReducer) {
      if (!isFunction(nextReducer)) {
          throw new Error('Expected the nextReducer to be a function.')
      }
}

I propose create local reducer and wrap state when get new action.
InitialState is default value for reducer.
This will make it convenient to receive a modified store.

[jednano]

See #145