MultiNodeSpec + Akka.Cluster bug fixes

Changes in this set:

* @smalldave has implemented most of the MultiNodeConfig and MultiNodeSpec - not totally done yet but most of the way there.
* Fixed a bug caused by new leaders being elected for a specific role after the cluster has already started.

Quick question about workflow. Fairly new to Git and PRs.
At the minute I'm submitting PRs to @Aaronontheweb fork. This works quite nicely because we are in different time zones. I submit PRs to him at the end of my day. He picks them up, does some more work and I pick up from his commits the next day.
Really I should just be submitting PRs to dev? That would allow me to fix these issues in the PR.
The problem is I want to build on work that is often sat in a PR. Do I merge / rebase the PR locally and just get on with it? 
What happens when I want to push / create a PR. Do I / can I add to existing PRs?