Add Cache-Control header for static assets.

This avoids wasting device cycles on retrieving the same data over
and over.

firmware/src/http.rs

@@ -174,6 +174,7 @@ pub fn write_reply_header(output: &mut fmt::Write, status: u16, content_type: &s
            status, status_text, content_type)?;
     if gzip {
         write!(output, "Content-Encoding: gzip\r\n")?;
+        write!(output, "Cache-Control: public, max-age=600\r\n")?;
     }
     write!(output, "\r\n")
 }