Store uncompressed AIPs

This is the sister PR to artefactual/archivematica#40.

This allows storing uncompressed AIPs via the storage service.

* Store AIP now works if no pointer file is present.
* Packages can now compress themselves, and the download API from the package resource will compress uncompressed packages before serving them.

Ignore the last commit here; it's duplicated from the other branch.
The temp dir this is compressed to should be deleted.  If the temp dir path can make it to ```utils.download_file_stream```'s temp_dir parameter, that will delete it.