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.
Decided I didn't like pushing this into `get_download_path` anyway (I definitely wouldn't expect `get_download_path` to churn away on 500 gigs of data!), so made it raise an exception in that case and had the resource handle compressing the file and passing the temp dir as appropriate.