PNG image export function
PNG Import/Export

Clicking "export" will now generate a PNG image
with voxel data embedded within it using the `lsb`
module.

Drag the image onto the window again to reload
the original voxel shape.
Ignore invalid imported images.
Removed unused argument for exportImage
Merge pull request #1 from hughsk/lsb-export

PNG Importer/Exporter