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

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)

Support time range search in more engines

add version info to config endpoint

serve pages with HTTP/1.1

[fix] ixquick : fetch results from ixquick.eu instead of ixquick.com

[fix] images modal with infinite scroll

[fix] seeks URLS, close #731

fix kickass torrents engine

[fix] set the title on the opensearch link tag

[engine] PDBe (Protein Data Bank Europe)