feat(lib): provide store functionality (resubmitted)

[derrickpelletier]

For #19

This started as a fork of #185.

What this does

• adds ability to provide an existing redux store instead of always creating a new one.
• when a store is provided, a local copy is made within ng-redux which is always updated with the full state of the primary store.
• dispatches to the ng-redux copy are proxied back to the primary store via middleware.

Notes

Credit to @AntJanus for getting the ball rolling on this. I couldn't think of a better solution to the problem, but open to suggestions.

@AntJanus, unlike your implementation, i stuck to always returning the ng-redux store and just proxying dispatches back up instead of returning the merge of the provided store with the patched in subscribe method from the ng-redux store. If this isn't optimal, let me know.

@deini, you were mentioned in the original PR for feedback, so mentioning you here again.

[derrickpelletier]

Small update:

1. added initial dispatch when wrapping the provided store to trigger pulling in the current state.
2. added support to the provided-store mode for middleware and enhacers.

[AntJanus]

Alright! I'm gonna merge this in :) Thank you for the hard work @derrickpelletier

[derrickpelletier]

Thanks @AntJanus! Can we get this released under an alpha or beta version tag on npm?