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/1158006/414322/980ff0ae-ac14-11e2-9897-1dc19c22f9b8.png)