Add small functionality to perceptron and graphical example

It makes it possible for the user to initialize w and bias of the perceptron. In this
way one can train several perceptrons for the same data and show that the
solution depends on the initial values of w and bias.

An example of the output given by the graphical example is:

![perceptron](https://f.cloud.github.com/assets/1158006/414014/aefa9a24-ac07-11e2-9dac-fceeab79f799.png)