[Python] Add convertLanguage(const char* language, int format) method.

This PR allows addon devs to convert one language format to another.
The method works as follows:
```
   /**
    *  convertLanguage(const char* language, int format)
    *  Returns the given language converted to the given format as a string.
    *
    * language: string either as name in English, 
    *           two letter code (ISO 639-1), 
    *           or three letter code (ISO 639-2/T(B)
    *
    * format: format of the returned language string
    *         0: full language name in English (default)
    *         1: two letter code as defined in ISO 639-1
    *         2: three letter code as defined in ISO 639-2/T or ISO 639-2/B
    *
    * example:
    *   - language = xbmc.convertLanguage(English, 2)
    *   - 'English' == xbmc.convertLanguage(English, 0) 
    */
```