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

This is useful not only for static content handling, so we could put it into server package.
And maybe find a better name, like RequestHandlerRouter or RequestHandlerDispatcher? 

Alternative implementation could follow the chain of responsibility pattern. Would be more generic, and applicable to other cases as well.