Add "Network status and alerts" page and RSS feed

Generate alerts pages through translate.rb plugin
Update sitemap plugin to include alerts

This pull request can be previewed live here : http://bitcoinalert.zapto.org/en/alerts

This pull request adds a "Network status and alerts" page on the bottom of the website with a RSS feed to allow people to follow or list network alerts. As well as to see when a network alert is ongoing or not.

Developers can add more alerts in _alerts directory, detailed instructions are in the README
https://github.com/bitcoin/bitcoin.org/tree/alerts#network-alerts

Alerts pages get simple urls such as : bitcoin.org/en/alert/dos . These urls allow us to translate alerts in the future if we have a responsive translators team.

![capture du 2013-05-26 16 10 29](https://f.cloud.github.com/assets/3578089/566009/4e0d04a6-c65e-11e2-8633-8a68b9a14d31.png)