A new drawSlice() method, and other changes

– Added a new drawSlice() method for creating pie charts more easily.
The usage for this method can be found in the documentation under the
'Slices' section.
– Fixed bug affecting the position of drawings drawn on multiple
canvases.
– Improved the efficiency of layer animation when multiple properties
are being animated
– The deprecated font property has been removed. Please use the
fontStyle, fontSize, and fontFamily properties instead
– Fixed a bug which affected the dragging of a layer if a mouseup
callback was present