[plt-scheme] s-expr differ

I wrote a little program to highlight the differences in s-expressions. 
Right now it only works in xterm but if anyone cares about it I can 
probably get it to work in other terms and drscheme.

Here it is working in xterm (yes my theme is xp):

http://www.cs.utah.edu/~rafkind/tmp/diff/diff.png

Code is there too

http://www.cs.utah.edu/~rafkind/tmp/diff/