Fix timing in example/classification scripts

[lukeyeager]

Previously, the scripts printed out a misleading message:

Processed 1/1 images ...
Classification took 0.0561039447784 seconds.

In fact, that value included more computation than just the net.forward() call.

Now, you get much more accurate timing information. You can see the difference between the first batch and any subsequent batch, for example:

Processed 1/1 images in 0.031234 seconds ...
Processed 1/1 images in 0.006955 seconds ...
Processed 1/1 images in 0.006529 seconds ...

Thanks for @flx42 for pointing this out!