io: 'dynamic' PipelineStage wrapper with tests, possibly fixes #198

Needed to be able to do START-TLS for my project, and this seemed to be the solution. 

Note. I just recently found Spray and still new to Scala and Akka as well, so I'm not at all sure its the correct way of doing this. But it seems to work.

My primary concern/worry is with the mutable variable of type Pipelines that's used to store the built pipeline stages contained within the dynamic wrapper. Assuming a built pipeline is only used from the same Actor it shouldn't be a problem though.