basic UNDO/REDO support, issue #24
prevent undo from removing contenteditable attribute, clean, rename and unify key event handeling
split up server-communications and diff-helpers
undo and redo by replaying rather than trying to undo patches (more stable), issue #24
undo and redo by replaying rather than trying to undo patches (more stable), second try, issue #24
smart undo/redo allowing unlimited amounts of undo-redo-undo-redo-undo-redo-..., solves issue #24