Validates presence of User#name

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>

doc/api/users.md

@@ -75,7 +75,7 @@ POST /users
 Parameters:
 + `email` (required)                  - Email
 + `password` (required)               - Password
-+ `name`                              - Name
++ `name` (required)                   - Name
 + `skype`                             - Skype ID
 + `linkedin`                          - Linkedin
 + `twitter`                           - Twitter account