Update ngInclude doc for quoted source

Request Type: docs

How to reproduce: 

Component(s): 

Impact: medium

Complexity: small

This issue is related to: 

**Detailed Description:**

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.

**Other Comments:**

In my fork I attempted to update the document file for ngInclude.  Probably due to my lack of experience I was unable to match the document source to what I see online at docs.angularjs.org//api/ng/directive/ngInclude.  Specifically I would have liked to update the Usage section to show ng-include="  'template.html' "  on all occurrences.  I've only experienced this issue using ngInclude as an HTML attribute, so I did not know if the Element and CSS class usage examples should also be updated.  Thanks for the great product.  Angular is awesome.