Use "immutable" Cache-Control extension

See http://bitsup.blogspot.com/2016/05/cache-control-immutable.html for
more info.

Fixes #103
Closes #119

modules/actions/serveStaticFile.js

@@ -15,7 +15,7 @@ function serveStaticFile(req, res) {
     .set({
       "Content-Length": req.entry.size,
       "Content-Type": getContentTypeHeader(req.entry.contentType),
-      "Cache-Control": "public,max-age=31536000", // 1 year
+      "Cache-Control": "public, max-age=31536000, immutable", // 1 year
       "Last-Modified": req.entry.lastModified,
       ETag: etag(req.entry.content),
       "Cache-Tag": tags.join(",")