Add ChannelFutureNotifier

ChannelFutureNotifier is added more for convenience: It's a simple extension
of FutureNotifier that additionaly holds a Channel it is associated with.
ChannelFutureNotifierWrapper was added as a simple wrapper that takes a
(Netty) ChannelFuture and exposes it as a ChannelFutureNotifier.
Add MockChannel

MockChannel provides a Channel that has been mocked with Mockito. This
Channel will keep a list of listeners that were added to the ChannelFuture
used for closing the Channel and informs them when close is called on
MockChannel.
Add MockPipeline