add UserSession.getCredential()

[jgravois]

first pass at #208

PRing against the amd branch for the sake of the demo.

require([
  "@esri/arcgis-rest-auth",
  "@esri/arcgis-rest-items",
  "esri/identity/IdentityManager",
  "esri/WebMap"
], function(
  arcgisAuth, arcgisItems, esriId, WebMap
) {
  const session = new arcgisAuth.UserSession({
    username: "arcgis_python",
    password: "P@ssword123"
  });

  // https://geosaurus.maps.arcgis.com/home/item.html?id=728043ac6a574a00b8f1cd5ee0eae8cd
  arcgisItems.getItemData("728043ac6a574a00b8f1cd5ee0eae8cd", { authentication: session })
    .then(response => {
      esriId.registerToken(session.getCredential());

      new WebMap({
        portalItem: {
          // dig the webmap id out of the response
          id: response.map.itemId
        }
      });
    });
});

[coveralls]

Coverage increased (+0.001%) to 99.326% when pulling c03430d on feat/jsapi-auth into 0cc7aaa on master.

[tomwayson]

not yet, so far we don't store multiple credentials.

[tomwayson]

I don't think so - let the consuming app decide if the half baked credential is valid for their use case or not

[jgravois]

tacked on another commit to use dojoConfig to identify the location of our UMDs instead of <script> tags and revert the rollup change.

then rebased.