fix startpage engine and add comments

* add language support
* remove not required code
* improve google-ad detection (no false detection anymore, I hope)
* other improvements
fix twitter engine and add comments

* add language-support
* add comments
* little refactoring
remove categories : social media from twitter engine in settings.yml
add little note to duckduckgo engine
fix wikipedia engine and add comments

* add paging support
* make number_of_results changable
* make result calculation more clear
* add comments