Akka v0.6.4 bugfix release

[Aaronontheweb]

I would like to do a bugfix / minor update release soon, mostly because we've fixed some pretty nasty bugs since the previous release two weeks ago.

I would like to do a new release once the following issues have been resolved: #350, #335, #331, #344

Any other issues that should be included? What about #292 ?

Notable breaking changes:

1. ExtendedActorSystem is now the accepted argument for actor system extensions

Anything else? Did we move anything out of the public interface and into internal that's worth noting?

[Aaronontheweb]

@akkadotnet/developers so, should we still try and put a new bugfix release out before we integrate the #299 changeset into the branch?

[HCanber]

If we put out a new release without #292 Namespace/Package Names we shouldn't publish Akka.Serilog (since we need to break the name in the future).
I think we can wait with #299 PriorityMailbox

[Aaronontheweb]

@HCanber I'll hold off on publishing the Serilog package.

[Aaronontheweb]

@HCanber I'll push updates today for all of the core Akka packages. The Akka.TestKit stuff we can push to NuGet as part of v0.7 #365 along with the Serilog package and PriorityMailbox.