导出 imageGroundLayer 文件；增加 .gitignore 文件

Author: xhwgood

.gitignore

@@ -0,0 +1 @@
+.DS_Store

tmap/index.d.ts

@@ -12,4 +12,5 @@
 /// <reference path="./overlay/multiLabel.d.ts" />
 /// <reference path="./overlay/domOverlay.d.ts" />
 /// <reference path="./overlay/infoWindow.d.ts" />
+/// <reference path="./overlay/imageGroundLayer.d.ts" />
 /// <reference path="./tools/geometryEditor.d.ts" />