The users API updated with return codes, e.g. if required parameters are missing
a `400 Bad Request` error is returned instead of `404`. Fixes return codes of functions,
e.g. deletion of a ssh key is an idempotent function now.
The API documentation is updated to reflect the current status of the API. Descriptions
are more detailed and complete, infos to return values are added to all functions.

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/