Use HttpServerDispatcher to fix remoteAddress property of Request.

Hi,

I wrote to [finatra-users](https://groups.google.com/forum/#!topic/finatra-users/5NAxoCtKdiQ) about the issue I had with hardcoded remoteAddress-property of Request. Today I had time to investigate and while browsing code for both Finatra and Finagle I noticed that just by changing FinatraServer to use [HttpServerDispatcher](https://github.com/twitter/finagle/blob/master/finagle-http/src/main/scala/com/twitter/finagle/http/codec/HttpServerDispatcher.scala) instead of [SerialServerDispatcher](https://github.com/twitter/finagle/blob/master/finagle-core/src/main/scala/com/twitter/finagle/dispatch/ServerDispatcher.scala) the property works.

Having just started with Finatra and Finagle, I decided to send this pull request to get some feedback if the change is ok.