Import of articles is implemented!

- Remove massiveImportAction and addCategories from FeedController
- Fix typo for some methods (camelCase)
- addCategoryObject and addFeedObject return id if corresponding object already
  exists in DB
- introduce addEntryObject. Return -1 if Entry already exist (in order to keep
  quite good performances)
- Complete importArticles method

Need some more tests + better performance