Added mouse support:

Hit a skill key, then click on a lemon to assign the skill.
(You can also click on nuke, help, pause, quit (I think) during play
and it will do the thing.)
Changed keyboard controls:
Now you must hit a skill key to select the current skill,
then put the cursor over a lemon and hit enter or space to assign
the skill.
The help screen still needs to be updated.