'TCPTransport' Refactoring

The 'TCPTransport' abstract class doesn't take the client actor as a
construction parameter anymore. The client is now supposed to be the
actor creating an instance of a 'TCPTransport' derived class. For the
specifications (Specs2) to work, some boilerplate is required to provide
a fake parent to this instance. see https://gist.github.com/archie/7952671
for more examples on that.
Asking 'coveralls' to ignore a line

Without this trick, the 'RemainingLengthCodec' will never be 100% covered
and still be tail recursive! To make it 100% covered without trick I'd
need to use 'flatMap' around a recursive call to 'decodeAux' making it non
tail recursive.