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

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

Googlers can find more info about SignCLA and this PR by following this link.

[Gum-Joe]

CLA has been signed.

[googlebot]

CLAs look good, thanks!

Googlers can find more info about SignCLA and this PR by following this link.