Adding author to make atom.xml generate properly

The [atom.xml](https://github.com/ddprrt/ddprrt.github.io/blob/master/atom.xml) uses `{{ site.author.name }}` and `{{ site.author.email }}`, which are not present in your config.

Thus, the generated atom file has this:

```
<author>
  <name/>
  <email/>
</author>
```

Adding three lines to `_config.yml`:

```
author:
  name: Stefan Baumgartner
  email: sbaumg@gmail.com
```