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

Does it make sense to just do a `doOnError()` so that you can avoid creating and managing an `Observable`. Specifically since the `errorMapper` are sync (returns void), it should just be fine?