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'd suggest an abstract base class that does some of the basics like sanitizing the input (unless I missed where that is happening) which could then be extended by the two existing impls you have.