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.
What about dbo:source (http://mappings.dbpedia.org/index.php/OntologyProperty:Source)? I'm not sure we need a completely new property to cover this, and it can be argued that the metadata document File:abc is derived from the source file. Do you think that makes sense?