New demo: random-noise: shows how well vowpal-wabbit can tell signal and noise apart

This is a demo that is good to demonstrate vw fundamental linear prediction and usage with a twist.

It learns from a random train-set that describes a (perfect) simple linear regressor with 3 input features.

Then it adds global noise to the train set.

Then it adds per-feature random noise to the train-set

In all 3 cases, predictions on an independent randomly-generated test-set are near perfect.

The demo is fully scripted, the user only needs to hit 'Enter' repeatedly.

Check the README file for full details.

A link to a presentation including an slide-annotated version of this demo can be found in the README.