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)

WARNING: dependency changes

Preferences class was introduced in order to handle user preferences. Right now
it parses cookies and the form in preferences. Also it can retrieve settings
based on the name of the setting.

ATTENTION
Please note that engine preferences are handled differently from now on. So it
introduces incompatible changes. Every user who has saved preferences should reset and
save his/her settings again.

This change was needed, because everytime a default disabled engine was
added saved user preferences would broke. Now engine setting tracking is
fixed.

implemented for wikipedia, qwant and google

Several changes has been made:
 - Parallel result merge
 - Scoring algorithm slightly changed (see result_score())
 - Proper Thread locking on global data manipulation