Add support for react router 3

[phpnode]

I just copied the solution from the redux router repo - reactjs/react-router-redux#476

[alisd23]

Thanks for this, surprised it's so easy! Could you also bump up the react-router version in the package.json to test it works with that version?

[alisd23]

Closing in favor of #3.

Decided to just bump a new major version (3 - in line with react-router) and stop supporting v2 because:

• The breaking changes from v2 to v3 aren't huge so devs really should be upgrading anyway.
• React Router won't be maintaining v2 anymore, it'll all be on v3.