load: use magic number to detect image format

[deltheil]

To solve #129 and problems like soumith/imagenet-multiGPU.torch#43.

I kept the old detection via the extension, but it should never been called (except for exotic JPEG files?). Of course we can as well remove it.

UPDATE: keeping the old detection is handy precisely if someones loads a non supported format, e.g.:

> image.load("test.gif")
<image.load> unknown image type: gif

[soumith]

Thanks. looks clean and simple.