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

Are we talking about a simple URI router much like discussed in issue #31 ?

If yes, I see it much like karyon's [SimpleUriRouter](https://github.com/Netflix/karyon/blob/master/karyon-core/src/main/java/com/netflix/karyon/transport/http/SimpleUriRouter.java)

I am for creating this utility to compose multiple `RequestHandler` as handling multiple URIs.