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 use case is different from what's implemented in SimpleUriRouter.  In SimpleUriRouter each uri prefix of regex is mapped to exactly one RequestHandler.  Here we can specify a cascading set of file sources that are inspected in a predefined order.  So, depending on which source contains the file the same URI may be end up using a different RequestHandler.  It would be equivalent to SimpleUriRouter mapping a URI to multiple RequestHandlers.