Remove extra `/` from the links from `feed.xml`

In the `feed.xml` file, the links to the archive posts contain one extra slash: `<link>http://wdrl.info//archive/[number]</link>`.

While this may not seem like an issue, in some contexts the extra slash can be problematic (e.g.: when adding the link from [Feedly](http://feedly.com/) to [Pocket](http://getpocket.com), [Pocket](http://getpocket.com) will just add `http://archive/[number]` instead of the correct link).

![](https://f.cloud.github.com/assets/1223565/2180965/eea158e6-973b-11e3-8f21-ccce38499d0e.png)

This commit also adds the missing `grunt-contrib-copy` module (Grunt complains that: `Local Npm module "grunt-contrib-copy" not found. Is it installed?`).