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.
Should be ``` if not self.is_compressed():```