Provide access to interpreted (translated) data

In fact, the interpreted data value as displayed by exiv2 is already accessible from pyexiv2, although quite hidden: the method _Image__getExifTagToString(key) returns this interpreted value as shown in the example below:

>>> image=pyexiv2.Image('xmptest.jpg')
>>> image.readMetadata()
>>> key = 'Exif.Image.Orientation'
>>> print key, ":", image[key], "/", image._Image__getExifTagToString(key)

    Exif.Image.Orientation : 2 / top, right

This method needs to be made more accessible, this bug will track the implementation of this functionality.

Implementation committed with revision 71.

Added method interpretedExifValue(key) to class Image that returns the interpreted value of an EXIF tag as displayed by the exiv2 command-line tool.
Here is an example of how to use it:

>>> import pyexiv2
>>> image = pyexiv2.Image('image.jpg')
>>> image.readMetadata()
>>> for key in image.exifKeys():
... print key, '=', image.interpretedExifValue(key)

Thanks!