Keep the gradient flowing

Reworked example gallery for scikit-learn

I've been working lately in improving the scikit-learn example gallery to show also a small thumbnail of the plotted result. Here is what the gallery looks like now:

And the real thing should be already displayed in the development-documentation. The next thing is to add a static image to those that don't generate any result, examples such as the SVM-GUI should have an image to display.