HTTP file server

Initial implementation for serving static content from RxNetty based on https://github.com/netty/netty/blob/master/example/src/main/java/io/netty/example/http/file/HttpStaticFileServerHandler.java.  

Additional features include,
1.  Support for single and multiple file sources
2.  zero-copy serving of files

Can this just be `Func2<Throwable, HttpServerResponse<I>, Observable<Void>>` or just `Func2<Throwable, HttpServerResponse<I>, Void>`?