add resize image w/u aspect ratio function

yolo/utils.py

@@ -0,0 +1,16 @@
+from PIL import Image
+
+
+def letterbox_image(image, size):
+    '''Resize image with unchanged aspect ratio using padding'''
+
+    img_width, img_height = image.size
+    w, h = size
+    scale = min(w / img_width, h / img_height)
+    nw = int(img_width * scale)
+    nh = int(img_height * scale)
+
+    image = image.resize((nw, nh), Image.BICUBIC)
+    new_image = Image.new('RGB', size, (128, 128, 128))
+    new_image.paste(image, ((w - nw) // 2, (h - nh) // 2))
+    return new_image