add ArcGIS MapServer token based security support

This adds support for ArcGIS's proprietary token-based authentication for
services. [1]

The solution is not robust: tokens are subject to expiration, after which point
the imagery provider will begin throwing exceptions (rightfully so).

Abstracting authentication further (outside of ArcGIS MapServer imagery
providers) may be a good idea, but is a larger task worth more thought.

Further, ArcGIS supports requesting security tokens for a set of credentials
via HTTP (GET, possibly POST as well), though this is not guaranteed to be
enabled on the server.

[1] - http://resources.arcgis.com/en/help/main/10.1/index.html#/About_ArcGIS_tokens/0155000005s0000000/

Source/Scene/ArcGisMapServerImageryProvider.js

@@ -56,6 +56,7 @@ define([
      *
      * @param {Object} options Object with the following properties:
      * @param {String} options.url The URL of the ArcGIS MapServer service.
+     * @param {String} [options.token] The ArcGIS token used to authenticate with the ArcGIS MapServer service.
      * @param {TileDiscardPolicy} [options.tileDiscardPolicy] The policy that determines if a tile
      *        is invalid and should be discarded.  If this value is not specified, a default
      *        {@link DiscardMissingTileImagePolicy} is used for tiled map servers, and a
@@ -117,6 +118,7 @@ define([
         //>>includeEnd('debug');
 
         this._url = options.url;
+        this._token = options.token;
         this._tileDiscardPolicy = options.tileDiscardPolicy;
         this._proxy = options.proxy;
 
@@ -205,10 +207,16 @@ define([
         }
 
         function requestMetadata() {
+            var parameters = {
+                f: 'json'
+            };
+
+            if (defined(that._token)) {
+                parameters.token = that._token;
+            }
+
             var metadata = jsonp(that._url, {
-                parameters : {
-                    f : 'json'
-                },
+                parameters : parameters,
                 proxy : that._proxy
             });
             when(metadata, metadataSuccess, metadataFailure);
@@ -244,6 +252,14 @@ define([
             }
         }
 
+        var token = imageryProvider._token;
+        if (defined(token)) {
+            if (url.indexOf('?') === -1) {
+                url += '?';
+            }
+            url += 'token=' + token;
+        }
+
         var proxy = imageryProvider._proxy;
         if (defined(proxy)) {
             url = proxy.getURL(url);
@@ -265,6 +281,18 @@ define([
             }
         },
 
+        /**
+         * Gets the ArcGIS token used to authenticate with the ArcGis MapServer service.
+         * @memberof ArcGisMapServerImageryProvider.prototype
+         * @type {String}
+         * @readonly
+         */
+        token : {
+            get : function() {
+                return this._token;
+            }
+        },
+
         /**
          * Gets the proxy used by this provider.
          * @memberof ArcGisMapServerImageryProvider.prototype
@@ -606,6 +634,10 @@ define([
             url += ':' + this._layers;
         }
 
+        if (defined(this._token)) {
+            url += '&token=' + this._token;
+        }
+
         if (defined(this._proxy)) {
             url = this._proxy.getURL(url);
         }