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

Won't doOnError still propagate the error to to the subscriber of Observable<Void>.  The idea here is to write the error response and not propagate the error to the container.