Only set updated for entries when present.

Fixes #22.
Revert "Run tests using nose and enable coverage."

This reverts commit 1a3adc848375c142a8bbc1e326141dc8e3231211.
Test improvements.

* Use smaller test feeds, better performance.
* Test for duplicate enclosure and content.
No duplication in enclosures and content.

Fix for #30.
Content extraction and some refactoring.
Factored out fetching and parsing.

This prevents additional fetching and parsing for each XPath.