Fix comparsion error:

2014-06-15T12:28:13.712401   [caching]       albums
Traceback (most recent call last):
  File "../photofloat/PhotoFloat/scanner/main.py", line 23, in <module>
    main()
  File "../photofloat/PhotoFloat/scanner/main.py", line 17, in main
    TreeWalker(sys.argv[1], sys.argv[2])
  File "/tmp/photofloat/PhotoFloat/scanner/TreeWalker.py", line 16, in __init__
    self.walk(self.album_path)
  File "/tmp/photofloat/PhotoFloat/scanner/TreeWalker.py", line 76, in walk
    album.cache(self.cache_path)
  File "/tmp/photofloat/PhotoFloat/scanner/PhotoAlbum.py", line 70, in cache
    self._sort()
  File "/tmp/photofloat/PhotoFloat/scanner/PhotoAlbum.py", line 53, in _sort
    self._photos.sort()
  File "/tmp/photofloat/PhotoFloat/scanner/PhotoAlbum.py", line 320, in __cmp__
    date_compare = cmp(self.date, other.date)
TypeError: can't compare datetime.datetime to unicode

Credits go to https://zfsftw.com/blogpost/80/DateTime_TypeError_-_Unicode