FileTypeExtractor for Commons files

This pull request adds a FileTypeExtractor that infers the rdf:type and dc:type of pages in the File: namespace on the Wikimedia Commons by using their extensions, and emits this information as triples. It closes gaurav#3.

This pull request also moves ImageExtractor's file and thumbnail URL generating code into ExtractorUtils and replaces them with Special:FilePath URLs as suggested in https://github.com/dbpedia/extraction-framework/issues/133#issuecomment-47937934 and so closes dbpedia#133 and dbpedia#139.