Update ngInclude doc for quoted source

As noted on stackoverflow.com (http://stackoverflow.com/questions/13943471/angularjs-ng-include), the use of ngInclude requires single quotes around the template filename.  The documentation and examples should show: <div ng-include="  'template.url'  "> instead of simply "template.url" to remind users to enclose the filename in single quotes.  As a additional step ngInclude should issue some warning that the include failed.  It now fails silently and makes for a bad day at the programming chair.