On OS X, read Exif tags using Cocoa's built-in functionality

This allows for RAW files Exif reading. Fixes #234.

cocoa/inter/app_pe.py

@@ -6,7 +6,6 @@
 # which should be included with this package. The terms are also available at 
 # http://www.hardcoded.net/licenses/bsd_license
 
-import os.path as op
 import plistlib
 import logging
 import re
@@ -48,6 +47,16 @@ class Photo(PhotoBase):
             raise IOError('The picture %s could not be read' % str(self.path))
         return blocks
     
+    def _get_exif_timestamp(self):
+        exifdata = proxy.readExifData_(str(self.path))
+        if exifdata:
+            try:
+                return exifdata['{Exif}']['DateTimeOriginal']
+            except KeyError:
+                return ''
+        else:
+            return ''
+    
 
 class IPhoto(Photo):
     def __init__(self, path, db_id):