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

This is used in FileResponses.setContentTypeHeader().

```java
    public static void setContentTypeHeader(HttpServerResponse<ByteBuf> response, File file) {
        MimetypesFileTypeMap mimeTypesMap = new MimetypesFileTypeMap();
        response.getHeaders().set(HttpHeaders.CONTENT_TYPE, mimeTypesMap.getContentType(file.getPath()));
    }
```
See documentation here, http://docs.oracle.com/javase/7/docs/api/javax/activation/MimetypesFileTypeMap.html