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

Purposes :
- isolate the plugins calls
- distinction between parsing the web request and running the search (Search class). To be able to test code easily, to run searx code outside a web server, to filter the search query parameters with plugins more easily, etc...

Details :
- request.request_data contains request.form or request.args (initialize inside pre_request() function)
- Query class is renamed RawTextQuery
- SearchQuery class defines all search parameters
- get_search_query_from_webapp create a SearchQuery instance (basically the previous Search.__init__ code)
- Search class and SearchWithPlugins class takes a SearchQuery instance as class constructor parameter
- SearchWithPlugins class inherites from Search class, and run plugins
- A dedicated function search_with_plugins executes plugins to have a well define locals() (which is used by the plugins code).
- All plugins code is executed inside the try...except block (webapp.py, index function)
- advanced_search HTTP parameter value stays in webapp.py (it is only part of UI)
- multiple calls to result_container.get_ordered_results() doesn't compute the order multiple time (note : this method was call only once before)
- paging value is stored in the result_container class (compute in the extend method)
- test about engine.suspend_end_time is done during search method call (instead of __init__)
- check that the format parameter value is one of these : html, rss, json, rss (before the html value was assumed but some text formatting wasn't not done)