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

I think this might be another place where it would be more clear if we had a good mechanism for building chains of handlers.  I think that is in essence what @elandau is trying to setup here.  But what each handler in the chain does with the request should be arbitrary.

This kind of gets back to the question of what is the difference between a chain of handlers and a pipeline and how we make both easy to configure and combine.