Akka v0.6.4 official release

This is only a maintenance release per #351 but it includes many changes to the codebase.

## Summary

1. Akka.TestKit is well on its way to becoming a stand-alone NuGet package, thanks to @HCanber 's hard work - it's not included in this release but will be available soon.
1. Serilog support was added to the contrib loggers - won't be released until #292 is resolved. kudos to @DamianReeves 
1. Lots of improvements and changes to make the F# API feel more like idiomatic F#, thanks to @Horusiath 
1. Memory footprint optimization for actors and actor paths. Thanks @rogeralsing and @smalldave 
1. Major bug fixes in Akka.Remote, namely fixing remote deployment, remote supervision, and transport-level bugs with outbound connections. Thanks to @rmiller1971, @smalldave and others for bringing these issues up.
1. First major steps towards Mono support have been added thanks to @mattnischan's contributions.
1. New router type, the `TailChoppingRouter`, has been added.

## Breaking Changes
* All `ActorSystem` extensions now take an `ExtendedActorSystem` as a dependency - all third party actor system extensions will need to update accordingly.
* Remote deployment in previous versions did not work as expected and major changes have been made to remote deployment paths and supervision modes. Please check your remote-deployed code accordingly after this update and report any issues here or on our Google Group.

## Relevant Links:
* [Akka.NET official website](http://akkadotnet.github.io/)
* [Akka.NET Google Group](https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!forum/akkadotnet-user-list)
* [Akka.NET on NuGet](http://www.nuget.org/packages/Akka/)