Finagle-based Kestrel spout

[rstrickland]

I intend to write a Kestrel spout based on Finagle, primarily so I can get the Finagle value-adds (such as tracing and stats) for free. If you're interested in me contributing this back, I will write it in Java and work to design it in such a way as to avoid code duplication with the Thrift spout. Otherwise I'll just write it in Scala using a more functional design.