The Apple dev specification:
    https://developer.apple.com/library/mac/documentation/QuickTime/QTFF/Metadata/Metadata.html

Basically the structure is like:
    |--meta
    |----hdlr
    |----keys
    |----ilst

1) The handler type in the metadata handler atom is ‘mdta’.
2) The key and value are stored separately for each key-value pair.
   The 'keys' atom stores the key table, while 'ilst' atom stores the
   values corresponding to the indices in the key table.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>