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.
As mentioned in person, this can be simplified to ```command = ['tar', 'c', '-C', relative_path, '-f', compressed_filename, basename]``` where ```relative_path = ''.join(full_path.rsplit(basename, 1))``` (replace once, but starting from the right).  You can update here, or I can update in the AIP reingest branch.