add voat.co engine

Simplify search.py

Sometimes openstreetmap returns with None as title. In these cases use an empty
string instead.

closes #758

set search language from settings.yml

also tried flask's flash feature but flask creates session cookies if it
isn't flushed. Avoiding session cookies to preserve privacy

closes #756
closes #761

see https://msdn.microsoft.com/en-us/library/ff795620.aspx for bing
specific search operators

closes #755

add lobste.rs xpath engine

The timeouts in settings.yml is about the total time (not only the HTTP request but also the prepare the request and parsing the response)
It was more or less the case before since the threaded_requests function ignores the thread after the timeout even the HTTP request is ended.

New / changed stats :
* page_load_time : record the HTTP request time
* page_load_count: the number of HTTP request
* engine_time : the execution total time of an engine
* engine_time_count : the number of "engine_time" measure

The avg response times in the preferences are the engine response time (engine_load_time / engine_load_count)

To sum up :
* Search.search() filters the engines that can't process the request
* Search.search() call search_multiple_requests function
* search_multiple_requests creates one thread per engine, each thread runs the search_one_request function
* search_one_request calls the request function, make the HTTP request, calls the response function, extends the result_container
* search_multiple_requests waits for the the thread to finish (or timeout)

Search architecture

- pre_search(request, search)
- post_search(request, search)
- on_result(request, search, result)

with
- request is the Flask request
- search a searx.Search instance
- result a searx result as usual