Feature/filetypeextractor

Added a FileTypeExtractor that can extract basic information about Commons files. This required moving the Image URL generation code from the ImageExtractor into ExtractorUtils.
Cool! What about the:
 - _wiki page_ dbo:source _jpeg file_
 - _jpeg file_ foaf:thumbnail _jpeg file - thumbnail_

thing? Should I replace foaf:thumbnail with dbo:thumbnail? If so, does it make more sense to do it as above or as:

 - _wiki page_ dbo:source _jpeg file_
 - _wiki page_ [dbo:thumbnail](http://mappings.dbpedia.org/index.php/OntologyProperty:Thumbnail) _jpeg file - thumbnail_