Support Etag mwEmbedLoader

@ranyefet looks like the Etag support was missing for mwEmbedLoader / AutoEmbed:

```bash
curl -I "http://kgit.html5video.org/tags/v2.0.2.rc4/mwEmbedLoader.php/wid/_243342/uiconf_id/12905712/entry_id/0_uka1msg4/playerId/1351566600?autoembed=true"
HTTP/1.1 200 OK
Date: Thu, 28 Nov 2013 18:56:58 GMT
Server: Apache/2.2.25 (Amazon)
X-Powered-By: PHP/5.3.27
Cache-Control: private, max-age=60
Expires: Thu, 28 Nov 2013 18:57:58 GMT
Connection: close
Content-Type: text/javascript
```

fixed in this pull request: 
```bash
curl -I "http://kgit.html5video.org/pulls/498/mwEmbedLoader.php/wid/_243342/uiconf_id/12905712/entry_id/0_uka1msg4/playerId/1351566600?autoembed=true"
HTTP/1.1 200 OK
Date: Thu, 28 Nov 2013 18:57:37 GMT
Server: Apache/2.2.25 (Amazon)
X-Powered-By: PHP/5.3.27
Etag: 81da17a8c6fa3895ca18b2dee6bbd06a
Cache-Control: private, max-age=53
Expires: Thu, 28 Nov 2013 18:58:31 GMT
Connection: close
Content-Type: text/javascript
```

Note both Etags hurt because of a 60 second ( or less? ) expire on server side generated KS. 

We should see about bumping that to a more reasonable amount of time like 24 hours like the default KS expire in startWidgetSession in the client lib.  

Clients that have shorter expires can always just set at embed time.