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

We should not use writeOnChannel() here as that adds the future of write to unflushed futures. This write does not need a flush.

Instead you can just use 

```java
ChannelFuture writeFuture = getChannel().write(region);
```