Everything is in packages now!

I finally got tired of scrolling though more files than could fit on my
screen. So far there are seven packages. The head package is
edu.emory.prove_it; its six subpackages are named expression,
sketch_canvas, statement_panel, theorem, util, and variable_panel. I
have tried to put everything in the most appropriate package.
(edu.emory.prove_it contains the main window and most of the code not
involved in one of its subpanels, as well as the Selectable interface
which is implemented by both drawables and expressions.