[Python] Extend xbmc.getLangauge to return various language formats.

This PR allows addon devs to specify the format of the returned language string.
The signature is
```C++
    /**
    * getLanguage([format], [region]) -- Returns the active language as a string.
    *
    * format: [opt] format of the returned language string
    *               xbmc.ISO_639_1: two letter code as defined in ISO 639-1
    *               xbmc.ISO_639_2: three letter code as defined in ISO 639-2/T or ISO 639-2/B
    *               xbmc.ENGLISH_NAME: full language name in English (default)
    *
    * region: [opt] append the region delimited by "-" of the language (setting)
    *               to the returned language string
    *
    * example:
    *   - language = xbmc.getLanguage(xbmc.ENGLISH_NAME)
    */
    String getLanguage(int format = CLangCodeExpander::ENGLISH_NAME, bool region = false);
```


I've taken the ConvertToTwoCharCode method from @pieh's repository.
Thanks @pieh! 

The feature request: http://forum.xbmc.org/showthread.php?tid=163023