Feature request: Inherit ImageMetadata from collections.MutableMapping

Hi,

would it be possible to make ImageMetadata inherit from collections.MutableMapping and provide the two missing class functions __len__ and __iter__. This way you get a bundle of nice features such as metadata.get(key, default), metadata.iteritems(), 'Some.Tag.Name' in metadata, [key for key in metadata] and so forth.

Example implementations for missing functions:
def __iter__(self):
    for key in self.exif_keys:
        yield key
    for key in self.iptc_keys:
        yield key
    for key in self.xmp_keys:
        yield key

def __len__(self):
    return len( [key for key in self] )