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.

First page now starts with 0 offset,
rather than starting on the 30th result.

DuckDuckGo returns 30 results on each page.

Following engines does not support "Last year":
 * Bing News
 * DeviantArt
 * DuckDuckGo
 * Yahoo
 * YouTube (noapi)

only works if both country and language are set

test_duckduckgo modified to reflect changes in duckduckgo's html