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.

If you switch to work off dev,as @Aaronontheweb say you will in the future, you'd be able to submit and fix PR:s yourself.

Updating the branch that you've created the PR from, will update the PR. So if you commit and push, these will appear on the PR. If you rebase and force push, the PR will be updated.

Right now, my best guess it you should fork Aarons branch, submit a PR to him. He merges your PR into `Aaronontheweb:akka-cluster` and the changes would appear on this PR.