added mode option at dropbox storage creation

allowing to choose between "dropbox" (access to full dropbox account)
and "sandbox" (cannot access files not created by the jio application)
mode.

examples/scenario.js

@@ -93,7 +93,8 @@
 //             type: "drivetojiomapping",
 //             sub_storage: {
 //               "type": "dropbox",
-//               "access_token" : "TOKEN"
+//               "access_token" : "TOKEN",
+//               "root" : "dropbox"      
 //             }
 //           }
 //         }

src/jio.storage/dropboxstorage.js

@@ -49,8 +49,12 @@
       throw new TypeError("Access Token' must be a string " +
                           "which contains more than one character.");
     }
+    if (typeof spec.root !== 'string' || !spec.root ||
+        (spec.root !== "dropbox" && spec.root !== "sandbox")) {
+      throw new TypeError("root must be 'dropbox' or 'sandbox'");
+    }
     this._access_token = spec.access_token;
-    this._root = "dropbox";
+    this._root = spec.root;
   }
 
   DropboxStorage.prototype.put = function (id, param) {

test/jio.storage/dropboxstorage.tests.js

@@ -20,7 +20,8 @@
   test("create storage", function () {
     var jio = jIO.createJIO({
       type: "dropbox",
-      access_token: token
+      access_token: token,
+      root : "dropbox"
     });
     equal(jio.__type, "dropbox");
     deepEqual(jio.__storage._access_token, token);
@@ -38,7 +39,8 @@
 
       this.jio = jIO.createJIO({
         type: "dropbox",
-        access_token: token
+        access_token: token,
+        root : "dropbox"
       });
     },
     teardown: function () {
@@ -166,7 +168,8 @@
 
       this.jio = jIO.createJIO({
         type: "dropbox",
-        access_token: token
+        access_token: token,
+        root : "dropbox"
       });
     },
     teardown: function () {
@@ -264,7 +267,8 @@
 
       this.jio = jIO.createJIO({
         type: "dropbox",
-        access_token: token
+        access_token: token,
+        root : "dropbox"
       });
     },
     teardown: function () {
@@ -366,7 +370,8 @@
 
       this.jio = jIO.createJIO({
         type: "dropbox",
-        access_token: token
+        access_token: token,
+        root : "dropbox"
       });
     },
     teardown: function () {
@@ -507,7 +512,8 @@
 
       this.jio = jIO.createJIO({
         type: "dropbox",
-        access_token: token
+        access_token: token,
+        root : "dropbox"
       });
     },
     teardown: function () {
@@ -679,7 +685,8 @@
 
       this.jio = jIO.createJIO({
         type: "dropbox",
-        access_token: token
+        access_token: token,
+        root : "dropbox"
       });
     },
     teardown: function () {
@@ -840,7 +847,8 @@
 
       this.jio = jIO.createJIO({
         type: "dropbox",
-        access_token: token
+        access_token: token,
+        root : "dropbox"
       });
     },
     teardown: function () {