Module collection

Collections are the container of member and media resources.

Collections are important to amplee as they will map eventually
to a physical container in the storage.

For instance a collection named 'notes' would map to:

- a table named 'notes' in a database
- a directory names 'notes' on the filesystem
- a bucket named 'notes' on Amazon S3

etc.

So it is very important to carefully choose the 'name_or_id' parameter
in the class AtomPubCollection