Also added tests.

The 2 reasons are :
- creation of user fail if name is empty : in after_save, it tries to create a
namespace with namespace.name = user.name and namespece validates presence
Namespace#name
- in the web app links to team members are broken with empty User#name because
they are of the form <a href'...'> user.name </a>

Didn't bother with files in db/, config/, or features/