SitemapSpider: Collect additional info in response.meta

I found myself often scraping data from a page which was actually readily available in the sitemap which I used to get to the page. Hence, I thought it would be nice to have that sort of information in the response which is parsed in my spider. Here a little example of what this patch does:
snippet from arstechnica sitemap:

```
	<url>
		<loc>http://arstechnica.com/gadgets/2013/08/microsoftgoogle-bring-back-the-good-youtube-windows-phone-app/</loc>
		<news:news>
			<news:publication>
				<news:name>Ars Technica</news:name>
				<news:language>en</news:language>
			</news:publication>
			<news:publication_date>2013-08-14T01:00:40+00:00</news:publication_date>
			<news:title>Microsoft/Google bring back the good YouTube Windows Phone app</news:title>
			<news:keywords>Gear &amp; Gadgets</news:keywords>
		</news:news>
		<lastmod>2013-08-14T16:01:19+00:00</lastmod>
		<changefreq>daily</changefreq>
		<priority>1.0</priority>
	</url>
```

and the response.meta which make it into the spider looks like this:

```
{'loc': 'http://arstechnica.com/gadgets/2013/08/microsoftgoogle-bring-back-the-good-youtube-windows-phone-app/',
'download_latency': 0.14206790924072266,
'publication': '',
'language': 'en',
'title': 'Microsoft/Google bring back the good YouTube Windows Phone app',
'download_timeout': 180,
'priority': '1.0',
'depth': 2,
'publication_date': '2013-08-14T01:00:40+00:00',
'keywords': 'Gear & Gadgets',
'news': '',
'lastmod': '2013-08-14T16:01:19+00:00',
'changefreq': 'daily',
'download_slot': 'arstechnica.com',
'name': 'Ars Technica'}
```

As you can see, the hierarchic from the xml document is not preserved but I figured that a sitemap has a strict enough and short enough grammar so that this should not pose any problems.

What do you think?