Add utils to print/display binary maker notes and other EXIF Undefined tags

[Hobson Lane]

Per discussion of Bug #942595 it would be nice to be able to display EXIF "Undefined" strings without garbling a terminal with control codes or ValueError faulting on non-ascii characters, e.g. ord()>128

Ideally, the following code should display human-readable text,

 exif = im.exif_keys
 for k in im.exif_keys:
  print "{0}: {1}".format(k,str(im[k].value))
 print '------------- Comment -------------------'
 print im.comment
 print '-----------------------------------------------'

However, these new utils methods won't help unless they are incorporated into the python string output interface for exif tag classes ( ExifTag.convert_to_python() ).

[Hobson Lane (hobs)]

Here's the patch utils.py that provides functions like

    undefined_to_human(python_string)
    latin_to_ascii(python_string)
    escape_unprintable(python_string)
    try_int(python_string)

The core exif.py string interface functions are unchanged. And try_int is an alternative more robust fix for Bug #942595, but again it too is not incorporated in the core exif.py. That's left as an exercise for the user (until a later patch or branch).

[Hobson Lane (hobs)]

Patch above is as "malformed" as the tags it's trying to protect against... it deletes utils.undefined_to_string() and leaves exif.py hanging out to dry. A better patch will follow shortly.

[Hobson Lane (hobs)]

Here's the patch that Oliver requested (I think). It only provides additional utilities for creating human-readable strings out of Undefined type tags like MakerNote.

    undefined_to_human(python_string)
    latin_to_ascii(python_string)
    escape_unprintable(python_string)
    try_int(python_string)

None of the existing pyexiv2 behavior is altered, so malformed tags (non ASCII bytes) can still cause trouble if printed.

[Hobson Lane (hobs)]

Here's the patch that actually incorporates undefined_to_human() into exif.py string conversion of Undefined type tags. Warning, this does change the output (for the better).

The output for included test images like smiley1.jpg are unchanged becasue they have no Undefined tags ( MakerNotes ) ...

    metadata = pyexiv2.ImageMetadata('test/data/smiley1.jpg')
    metadata.read()
    for k in metadata.exif_keys():
        print metadata[k]

Works just as before