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.
Sounds good! ImageExtractor turns out to do three things:
 - _wikipage_ foaf:depiction _image_
 - _wikipage_ dbo:thumbnail _image-thumbnail_
 - _image_ foaf:thumbnail _image-thumbnail_

To that, I've added one more:
 - _wikipage_ dbo:source _image_

All of this is done in @9ce7c60! Once I finish figuring out how to generate thumbnail URLs for each file type (see @70d5a09249c for a first attempt at that), this pull request will be ready for your perusal. I'll work on that tomorrow.