languages.py can change, so users may query on a language that is not
on the list anymore, even if it is still recognized by a few engines.

also made no and nb the same because they seem to return the same,
though most engines will only support one or the other.

If no language is specified, bing returns results with multiple languages
for one query which isn't really useful. Setting english as default
insted if nothing.

and refactor method to make it testable without making requests

utils/fetch_languages.py gets languages supported by each engine and
generates engines_languages.json with each engine's supported language.

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

closes #755