Per-core CPU graphs

By passing core=0, core=1, etc parameters to the widget.

The default value (core="all") is still to show all cores
CPUGraph: Lag compensation

If the update interval is higher than what it should be, push the same
value several times to the graph to compensate this lag.

Every value shown by the cpu graph is actually an average since the last
lookup, so this actually gives nice average values!
Fix typo

Probably should remember what's passed into the constructor. Thanks @dequis for
always whining :-)