Add namespace_id to project creation for a user in API
Created by: meowstars
This patch just adds the possibility to change the namespace of a project created for a user via the API.
It behaves like https://github.com/gitlabhq/gitlabhq/pull/3119.
It might be interesting, for an administrator, to create projects in the global namespace for specific users. This is possible when we create a project without specifying the owner so why not here.