Added recursive image searching to the retrainer

[Gum-Joe]

When using the retrainer script, only the first level of directories is searched. So, given this file tree:

images
   class_1
      image.jpg
   class_2
      sub_class_3
         image2.jpg

Only image.jpg would be detected, not image2.jpg
This can be troublesome, for example, when you want an image to also fall into a more broad category, i.e category bottoms, sub-category jeans for a clothing classifier.

I've edited the functioncreate_image_lists to now recursively scan directories for photos, joining categories together with a ..
For example, given this file tree:

images
   tops
      t-shirts
         superdry
            t-shirt.jpg
   bottoms
      jeans
            jeans.jpg

t-shirt.jpg would be labelled tops.t-shirts.superdry and jeans.jpg as bottoms.jeans

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