Add a Granite UI DataSource for Generic Lists

This pull request adds a custom GraniteUI datasource for use with GenericLists.

To use this, for example, with the Granite UI Select component:

    {
      "jcr:primaryType": "nt:unstructured",
      "name": "./social",
      "fieldLabel": "Social Service",
      "sling:resourceType": "granite/ui/components/foundation/form/select",
      "datasource": {
        "jcr:primaryType": "nt:unstructured",
        "path": "/etc/acs-commons/lists/sharethis-services",
        "sling:resourceType": "acs-commons/components/utilities/genericlist/datasource"
      }
    }