Add usage properties to directories and users

[a-surfer]

In order to display the user's usage in the user's profile, we are calculating the space that each directory takes. Then we have the URL to allow a given user to query for her usage: https://beehub-devel.grid.sara.nl/system/users/john_doe?usage. That URL returns a JSON-formatted bundle with the directories' sizes.

Since we already have written the functionality to do the size calculation, we should also make it available via WebDAV. We can include such a BeeHub property on directories and on users.