Add link to download pointer files

Added a link to download pointer files associated with a Package, if they exist.  Refactored extract and download request code to pull out the section that returns a file stream, for use with the pointer file
downloads.  Also moved extract files to the Package, instead of in the resource.

#4 should be merged first, as this branch is based off it.  Supports artefactual/archivematica#25