This PR fixes the result count from bing which was throwing an (hidden) error and add a validation to avoid reading more results than avalaible.
For example :
If there is 100 results from some search and we try to get results from 120 to 130, Bing will send back the results from 0 to 10 and no error. If we compare results count with the first parameter of the request we can avoid this "invalid" results.
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.
