Add match_language function in utils to match any user given
language code with a list of engine's supported languages.

Also add language_aliases dict on each engine to translate
standard language codes into the custom codes used by the engine.

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.

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

closes #755