Add images attachement simple support

You must specify your application Host:port exemple EmulatorUtil.setGlasswareHost(host). By default is http://localhost:8080

You must declare a new Servlet in your grasware to download attachements :
 <servlet>
    <servlet-name>DownloadAttachementsServlet</servlet-name>
    <servlet-class>com.google.api.services.mirror.utils.DownloadAttachementsServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>DownloadAttachementsServlet</servlet-name>
    <url-pattern>/attachements/*</url-pattern>
  </servlet-mapping>