Display line numbers for the file being edited


![image](https://f.cloud.github.com/assets/5099049/1417392/a70a8e9e-3f89-11e3-8859-fa8e17787d7d.png)


Adds a JTextPanel to the left of the TextAreaPainter in the JEditTextArea.

Stores this in a new "LEFT" component of the JEditTextArea's ScrollLayout.

The new line number component only updates and repaints when one of the parameters changes (the displayed line numbers of the panel width (which is based on the display width of the maximum number).

The line number panel may update whenever the updateScrollBars() or painter.repaint() methods are called.