[example] add fully-convolutional efficiency note + confidence map

- spell out fully-convolutional efficiency
- add confidence map
- fix input size: 451 x 451 is correct for an 8 x 8 output map by the
  equation input size = 227 + 32(d-1) for output map dimension of d

examples/imagenet/imagenet_full_conv.prototxt

@@ -2,8 +2,8 @@ name: "CaffeNetConv"
 input: "data"
 input_dim: 1
 input_dim: 3
-input_dim: 454
-input_dim: 454
+input_dim: 451
+input_dim: 451
 layers {
   name: "conv1"
   type: CONVOLUTION