Added a class on selector

Added a class on the selector so that only expected elements show up in result.

Considering page http://www.dmoz.org/Computers/Programming/Languages/Python/Books/.
We expect "An Introduction to Python" to be the first entry in our result, as per the requirement stated at [scrapy tutorial](http://doc.scrapy.org/en/latest/intro/tutorial.html#intro-tutorial). However the first entry we were getting is "Website: name=[u'Top'] url=[u'/']" which should not be the case.
So, this fix.