implement swisscows engine

TODO: implement image engine

- Web
- Images
- News
- Social media

The API needs an API_KEY
The NOAPI doesn't have the published dates.

This allow to send request to google.* (according to the search language).
Before this commit, request in other languages than english was sent to www.google.com which was redirected to www.google.*
The PREF is still use on the www.google.com domain.

[fix] flickr engine

[fix] bing_news - fixes #328

[enh] currency_convert engine : "1 dollars in euros"

The currency names are fetched wikidata and store into a static file : searx/data/currencies.json
This file is loaded when the currency_converter is loaded.
A database is perhaps more appropiated.

[fix] yahoo_news engine

update versions.cfg to use the current up-to-date packages

[fix] twitter engine

[fix] Flickr engine

[enh] reduce the number of http outgoing connections.

Revert of #195 when the search language is not english

Sometimes there is two requests to google (depending of the source IP) : one to google.com, the second to google.fr (for instance).

Going to https://www.google.com/ncr and saving the PREF cookie for future use prevent this (there is no redirection).

But, recently (or not ?), by doing this the search returns English results even if the Accept-Language is specified.

There is still a way to prevent this : going to preference, set the search language. I don't know if this can be done by searx.

For now, a quick fix is to disable the use of the PREF cookie when the search language is not English (google engine will slower but returns excepted results).

Conflicts:
	searx/templates/oscar/preferences.html