Feature 3 - Integration

First attempt of using styles in android ListView.
Adding the functionality to click the notification displays the downloaded pdf.
Improvements in the exclusion of an article downloaded. 
Refactoring download service to write the file to the memory card. Thought behind the implementation described in http://developer.android.com/guide/topics/data/data-storage.html # filesExternal
Adding contextual menu in the list of downloaded items.

Changes overshadowed by daniel change. Will be resolved later.