Support modified date for RSS

RSS does not know the modified date for items.

However, it's possible to use other namespaces providing a modified item property.

```
<dc:modified>2013-09-01T06:07:58+02:00</dc:modified>
<atom:modified>2013-09-01T06:07:58+02:00</atom:modified>
```

1. I've added the modified date to Dublin Core handler.
2. I've added a new Atom handler reading the modified date.
3. Fixed some typos.

BTW: I could not run automatic tests since there is a problem with my Eclipse 4.3 and RSS tests.

It would be great if RSSOwl, my daily RSS newreader, would support modified dates for RSS items.