Delegate the loading of tweets to the client

Also fixes the problems caused by https://github.com/nerdzeu/nerdz.eu/commit/8a5e733a2135e644e317fa71693abaa90d5631fe.

Since the loading of tweets is now client-side, you must remove the `tmp/twitter_cache` folder.

**NOTE**: this commit adds a workaround for an issue found with the `[twitter]` image `onload` handler, which involves a small patch to the `messages` class. You can find more details in the Javascript class file.