Type validation for configs

Changes in this PR:

* When resolving `ActorRefProvider` from the HOCON config, we now validate that the type exist and is a  `ActorRefProvider` sub type

Issue found here https://twitter.com/kot_2010/status/492228000411561984/photo/1 .. double `"` is parsed as `""` + `Akka.Remote.RemoteActorRefProvider` + `,` //<- json terminate line
So "" is not a syntax error, it just means empty string and then HOCON unquoted string literal parsing kicks in.. so not a parser error but a malformed config.

* Added task-dispatcher to default config
> `TaskDispatcher` is slightly slower than our default dispatcher, but works under medium trust.