change class name from ArbitrarySetting to UserSetting

according to spec and comment
rename "key" to "label", according to spec
understanding the spec: rename "label" to "field_name"
add new field "label", for the ReST API

It is not used in the dot notation
add new field "type", for the ReST API

It is not used in the dot notation, and there's no type casting for now
API get

GET settings/<user.pk> => return the JSON object containing all the settings