[enh] Add onions category with Ahmia, Not Evil and Torch

Xpath engine and results template changed to account for the fact that
archive.org doesn't cache .onions, though some onion engines migth have
their own cache.

Disabled by default. Can be enabled by setting the SOCKS proxies to
wherever Tor is listening and setting using_tor_proxy as True.

Requires Tor and updating packages.

To avoid manually adding the timeout on each engine, you can set
extra_proxy_timeout to account for Tor's (or whatever proxy used) extra
time.

searx/engines/ahmia.py

@@ -0,0 +1,82 @@
+"""
+ Ahmia (Onions)
+
+ @website      http://msydqstlz2kzerdg.onion
+ @provides-api no
+
+ @using-api    no
+ @results      HTML
+ @stable       no
+ @parse        url, title, content
+"""
+
+from urllib.parse import urlencode, urlparse, parse_qs
+from lxml.html import fromstring
+from searx.engines.xpath import extract_url, extract_text
+
+# engine config
+categories = ['onions']
+paging = True
+page_size = 10
+
+# search url
+search_url = 'http://msydqstlz2kzerdg.onion/search/?{query}'
+time_range_support = True
+time_range_dict = {'day': 1,
+                   'week': 7,
+                   'month': 30}
+
+# xpaths
+results_xpath = '//li[@class="result"]'
+url_xpath = './h4/a/@href'
+title_xpath = './h4/a[1]'
+content_xpath = './/p[1]'
+correction_xpath = '//*[@id="didYouMean"]//a'
+number_of_results_xpath = '//*[@id="totalResults"]'
+
+
+def request(query, params):
+    params['url'] = search_url.format(query=urlencode({'q': query}))
+
+    if params['time_range'] in time_range_dict:
+        params['url'] += '&' + urlencode({'d': time_range_dict[params['time_range']]})
+
+    return params
+
+
+def response(resp):
+    results = []
+    dom = fromstring(resp.text)
+
+    # trim results so there's not way too many at once
+    first_result_index = page_size * (resp.search_params.get('pageno', 1) - 1)
+    all_results = dom.xpath(results_xpath)
+    trimmed_results = all_results[first_result_index:first_result_index + page_size]
+
+    # get results
+    for result in trimmed_results:
+        # remove ahmia url and extract the actual url for the result
+        raw_url = extract_url(result.xpath(url_xpath), search_url)
+        cleaned_url = parse_qs(urlparse(raw_url).query).get('redirect_url', [''])[0]
+
+        title = extract_text(result.xpath(title_xpath))
+        content = extract_text(result.xpath(content_xpath))
+
+        results.append({'url': cleaned_url,
+                        'title': title,
+                        'content': content,
+                        'is_onion': True})
+
+    # get spelling corrections
+    for correction in dom.xpath(correction_xpath):
+        results.append({'correction': extract_text(correction)})
+
+    # get number of results
+    number_of_results = dom.xpath(number_of_results_xpath)
+    if number_of_results:
+        try:
+            results.append({'number_of_results': int(extract_text(number_of_results))})
+        except:
+            pass
+
+    return results