Add portName to 'storeReady' message

[sokki]

First of all: thank you so much for this library ❤️

We have multiple redux stores that we want to sync. That works mostly great with the portName parameter. But we run into race conditions regarding the storeReady message: Whatever store is ready first, triggers a storeReady for all stores because the storeReady did not take the portName into consideration.

[tshaddix]

I'm glad this library is adding value to your projects!

This PR looks great! Thank you very much for contributing! Really appreciate the work you've done here (especially the tests!)