Implementation of the Inbox feature

Linked to issue #200.

I've decided to do some changes comparing to original implementation:

- Original `Inbox.Select` now is called `Inbox.ReceiveWhere` and uses standard .NET Predicate<object> instead of Scala'ish predicates (which are able to handle both condition checking AND object-to-object mapping of values). I've decided to change name to reflect current meaning of the method, since Select is a popular LINQ method which works in different way.
- Inbox doesn't use ActorDSL (which is probably not implemented yet in Akka.NET). This also reflects in configuration node - _akka.actor.inbox.*_ instead of original _akka.actor.dsl.*_ (see: Pidgeon.conf changes).
- Inbox queue has it's own custom implementation, based on LinkedList instead of arrays (like original .NET queues). This is because of necessity to implement `DequeueFirstOrDefault` method (which dequeues element meeting provided predicate, even if that element is not the at the beginning of the queue) required to implement selective inbox receives.

I've also tried to implement an `IObservable` interface, but since Inbox works in request-response manner and not a push-notification, it would require more significant modifications.