BAC-1251: handle If-None-Match and disable the support of If-Modified-Since

The following behavior will be enabled when ``$wgUseETag = true``

```
$ curl -x dev-macbre:80 -I "poznan.macbre.wikia-dev.com/wiki/Gzik" -H "If-None-Match: 20140120130416-123456"
HTTP/1.1 304 Not Modified
ETag: 20140120130416-123456

$ curl -x dev-macbre:80 -I "poznan.macbre.wikia-dev.com/wiki/Gzik" -H "If-Modified-Since: Mon, 20 Jan 2014 13:04:16 GMT"
HTTP/1.1 200 OK
ETag: 20140120130416-123456
Last-Modified: Mon, 20 Jan 2014 13:04:16 GMT
```

@prein / @michalroszka