Fix NPE when URL is incorrect
Use date from items if not in feed

With some RSS feeds, global publish date is not set.
We can use in place the most recent item date:

```xml
<rss version="2.0">
    <channel>
        <title></title>
        <link></link>
        <description></description>
        <item>
            <title></title>
            <link></link>
            <pubDate>Mon, 10 Mar 2014 16:57 EDT</pubDate>
            <description></description>
        </item>
    </channel>
</rss>
```

Closes #29.
Refactor tests

We want to optimize some tests. All integration tests will now use the same *random* cluster. So adding new tests won't slow down the overall release process.

In addition to this, all integration tests will now use local rss files instead of relying of the network connectivity.

Closes #30.