Installs Akka.TestKit.TestEventListener as default logger for TestKit

The default Akka.Event.DefaultLogger is replaced by Akka.TestKit.TestEventListener
This logger behaves exactly like DefaultLogger, but makes it possible
to use EventFiltering. If no filter is specified it logs to StdOut just like
DefaultLogger.
Discussed in issue #323
Remove EventFilter from AkkaSpec and use TestKit's
Refactor Mailbox.Post to include Receiver

Old: void Post(Envelope envelope)
New: void Post(ActorRef receiver, Envelope envelope)

This will make for better DeadLetter logging as DeadLetterMailbox (which is swapped in for terminated actors) now can include the intended recipient (i.e. the terminated actor).
Rename GetMillisDuration to GetTimeSpan and provide Obsolete-marked fallback

This is a non breaking change. GetMillisDuration still exists but has been marked as Obsolete.
Merge pull request #408 from HCanber/rename-GetMillisDuration-332

Rename GetMillisDuration to GetTimeSpan and provide Obsolete-marked fallback
Merge pull request #407 from HCanber/mailbox-post-include-receiver

Refactor Mailbox.Post to include Receiver
Merge pull request #412 from HCanber/testeventlistener-default-logger-323

Installs Akka.TestKit.TestEventListener as default logger for TestKit
Logging: rename arg "IActorContext cell" to "IActorContext context"

Reason: Looked strange from the outside, that I have to prrovide a cell when I do not have access to it.

Also create private const for LogLevel strings that's currently repeated twice.
Add documentation to SetReceiveTimeout() and ReceiveTimeout
Merge pull request #415 from HCanber/receive-timeout-doc

Add documentation to SetReceiveTimeout() and ReceiveTimeout
Merge pull request #414 from HCanber/logging

Logging: Rename argument "IActorContext cell" to "IActorContext context"
Merge pull request #413 from HCanber/remove-akkaspec-eventfilter-377

Remove AkkaSpec's EventFilter
TestActorRef should not be an actual ActorRef

Instead it mimics an ActorRef, but uses composition instead of inheritance.
With implicit operator that casts a TestActorRef to an ActorRef and overloaded == and != operator,  the user will probably not notice that it's not an actorref
Move Equivalent from XunitAssertions to XAssert.

Rename XUnitHelpers.cs to XAssert.cs
Incorporate TestBarrier into TestKit

Use CreateTestBarrier() inside a test class to create a new TestBarrier
Make TestKit non abstract and everything public.

This will enable usages when the user do not wish to inherit from TestKit but instead create a TestKit instance, assign it to a variable and perform tests on it.
Move files from src/core/Akka.Testkit.Tests to src/core/Akka.TestKit.Tests
Make TestKit an ActorRefFactory and add ActorOfAsTestActorRef

Remove TestActorRef.Create helpers
Remove obsolete members from TestKit
Moved classes from Internals namespace to Internal