Added JTree to TreeViewer.

Added a JTree representing ANTLR's parse tree of the input. When selecting a certain node, `mult_expr` for example:

![enter image description here][1]

the tree on the right gets updated to that particular tree. When pressing the <kbd>PNG</kbd> button, only that tree will be exported to a PNG image.

  [1]: http://i.imgur.com/GYOwiEM.png