Add support for Finagle HTTP servers

This pull request adds preliminary support for Finagle HTTP. It provides constructors and Clojure-friendly wrappers for `Server`, `Request`, and `Response` objects, and basic `ServerBuilder` support. It does not yet attempt to wrap all possible getters and setters.
My understanding is that both of the forms I've wrapped actually do return a `MockRequest`. See https://github.com/twitter/finagle/blob/master/finagle-http/src/main/scala/com/twitter/finagle/http/Request.scala#L195