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

By reading code it appears that netty will close the file after writing it on the channel. 

Specifically, the method [DefaultFileRegion.deallocate()](https://github.com/netty/netty/blob/master/example/src/main/java/io/netty/example/file/FileServerHandler.java) will be called by [ChannelOutboundBuffer](https://github.com/netty/netty/blob/master/transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java#L246) post transfer. So, I think we can safely remove this close handler.