More support for the unified dictionary interface.

Addded fromDict() function to ID3v2Tag. Added fromDict() and
toDict() functions to the TagUnion class (uses the first non-empty tag).
Added fromDict() and toDict() functions for the generic Tag class, only
handling common tags without duplicates. Addded preliminary mp3 test
case. Python3 bindings now available on my github site.

taglib/tag.h

@@ -58,6 +58,22 @@ namespace TagLib {
      */
     virtual ~Tag();
 
+    /*!
+     * Unified tag dictionary interface -- export function. Converts the tags
+     * of the specific metadata format into a "human-readable" map of strings
+     * to lists of strings, being as precise as possible.
+     */
+    virtual TagDict toDict() const;
+
+    /*!
+     * Unified tag dictionary interface -- import function. Converts a map
+     * of strings to stringslists into the specific metadata format. Note that
+     * not all formats can store arbitrary tags and values, so data might
+     * be lost by this operation. Especially the default implementation handles
+     * only single values of the default tags specified in this class.
+     */
+    virtual void fromDict(const TagDict &);
+
     /*!
      * Returns the track name; if no track name is present in the tag
      * String::null will be returned.