extended HtmlXPathSelector adding extract_links method 

It imitates the ```iterlinks``` method from ```lxml.html```, except for the style link extraction. Maybe we can extend it a little more and add the common case if ```onclick``` with ```window.location```.

```python
>>> from scrapy.contrib.selector.html import HtmlXPathSelector
>>> hxs = HtmlXPathSelector(text='<a href="alink">LOLOLOLO</a>')
>>> hxs.extract_links()
[u'alink']
```

Useful link http://www.w3.org/TR/REC-html40/index/attributes.html