Add support for GetLanguageNames API method

This is the API method documented [here](http://msdn.microsoft.com/en-us/library/ff512414.aspx). It is used like so:

``` ruby
bing.language_names(['en', 'es', 'de']) #=> ['English', 'Spanish', 'German']
bing.language_names(['en', 'es', 'de'], 'de') #=> ['Englisch', 'Spanisch', 'Deutsch']
```