API: list of namespaces can be retrieved for user

The list of namespaces can be retrieved for a given user. This is useful to check
in which namespaces projects can be created. At the moment a user or admin can only
create projects in different namespaces via web client, not in the API.
API: project creation allows namespace id, also access to namespaces

Currently there is no good way to see which namespaces a user is allowed to create projects
in. It is now possible to create a project with a valid namespace id, e.g. as administrator
to create a project in the global namespace or a user is allowed to create a project within
a group.

Tests added for access of namespaces via API.
Changed API entity definition of Groups, inherits from Namespace
Fixes parameter wiki_enabled in project creation

Fixes key `wiki_enabled` in parameters hash used when creating project.
API: fixed tests & added new ones to check project creation with namespaces

Tests check project creation with namespaces, e.g. if project can be created for a group
or as an administrator. Fixed a few tests.
API: documentation for namespaces added

Documentation for namespaces API added, with short descriptions and list of possible return codes.
Merge branch 'master' into feature/api_namespaces
Merge remote-tracking branch 'origin/feature/api_namespaces' into clients/sys

Conflicts:
	lib/api/projects.rb
	spec/requests/api/projects_spec.rb