Add support for Finagle Framework

[Finagle](http://twitter.github.io/finagle/) is a reactive scala framework for RPC. It is internally based on netty.
Finagle uses [future composition](https://twitter.github.io/finagle/guide/Futures.html) for building processing logic and therefore doesn't work well with our standard thread-based context propagation.

In order to provide a useful instrumentation, we need to cover at least the following features:
- [ ] Implement context propagation across finagle futures (most likely via [contexts](https://twitter.github.io/finagle/guide/Contexts.html) )
- [ ] Add instrumentation for the finagle HTTP server
- [x] Add instrumentation for the finagle HTTP client