fixed eating fish and fish killer achievement
fixed 'all poison/growth upgrades bought' achievements
fixed size and positioning of buttons
fixed murky water upgrade
added comprehensive bug and todo list
started to fix poison
some debugging done
useful debug info added
removed dependency on painter
simplified cursor switching
refactored countdown
improved variable names
modularised some functionality
moved population code out of level.js
tidied up some comments
POISON IS FIXED
updated readme to reflect poison fix
fixed score update on upgrade purchase
removed upgrade dependency on level
clarified readme.md