[USTABLE] ActorSystem Extensions, Remoting, RemoteTransport, EndpointManager

Almost there with issue #1

* Built upon what @rogerasling started with for ActorSystem extensions and made some changes so they can be loaded via config / static helper methods. Actively using them in a couple of places right now inside Akka.Remote, like the `AddressUidExtension` - added some specs inside `ActorSystemSpecs` to validate
* Finished modifying the `RemoteTransport` in order to bring it in-line with Akka
* Finished implementing the `EndpointManager` and `Remoting` - there aren't very many specs to cover them other than `RemotingSpec` at the moment, so I suspect there are bugs to squash here still.

Starting work now on the `RemoteConfigSpec` to validate that all of the settings I need to enable `TcpTransport` are there, and then it's onto implementing `TcpTransport`!!!