Add initial Gearman-based extractor module:

* Add MediaWords::DBI::Downloads::extract_and_vector() subroutine with self-contained "extract or store the error in database" code
* Add MediaWords::GearmanFunction::ExtractAndVector Gearman worker to be used for local and Gearman-based extractions
* Move "mediawords_extract_and_vector.pl" to "mediawords_extract_and_vector_locally.pl" to specify what the script is doing
* Add "mediawords_enqueue_unextracted_downloads_on_gearman.pl" helper script