fix stackoverflow and add comments

## Stackoverflow (It)

# 

# @website     https://stackoverflow.com/

# @provide-api not clear (https://api.stackexchange.com/docs/advanced-search)

# 

# @using-api   no

# @results     HTML

# @stable      no (HTML can change)

# @parse       url, title, content

from urlparse import urljoin

from cgi import escape

from urllib import urlencode

from lxml import html

# engine dependent config

categories = ['it']

paging = True

# search-url

url = 'http://stackoverflow.com/'

search_url = url+'search?{query}&page={pageno}'

result_xpath = './/div[@class="excerpt"]//text()'

paging = True

# specific xpath variables

results_xpath = '//div[contains(@class,"question-summary")]'

link_xpath = './/div[@class="result-link"]//a|.//div[@class="summary"]//h3//a'

title_xpath = './/text()'

content_xpath = './/div[@class="excerpt"]//text()'

# do search-request

def request(query, params):

    params['url'] = search_url.format(query=urlencode({'q': query}),

                                      pageno=params['pageno'])

    return params

# get response from search-request

def response(resp):

    results = []

    dom = html.fromstring(resp.text)

    for result in dom.xpath('//div[@class="question-summary search-result"]'):

        link = result.xpath('.//div[@class="result-link"]//a')[0]

    # parse results

    for result in dom.xpath(results_xpath):

        link = result.xpath(link_xpath)[0]

        href = urljoin(url, link.attrib.get('href'))

        title = escape(' '.join(link.xpath('.//text()')))

        content = escape(' '.join(result.xpath(result_xpath)))

        results.append({'url': href, 'title': title, 'content': content})

        title = escape(' '.join(link.xpath(title_xpath)))

        content = escape(' '.join(result.xpath(content_xpath)))

        # append result

        results.append({'url': href, 

                        'title': title, 

                        'content': content})

    # return results

    return results

  - name : stackoverflow

    engine : stackoverflow

    categories : it

    shortcut : st

  - name : startpage