PNG Importer/Exporter

Here's a PNG export/import for voxel-painter using [lsb](/hughsk/lsb) :)

Currently, the export button will open a new window with the image, and you can drag it back into the window to reload the saved data. Should be easy enough to change though - `importImage` only needs an image element (returning `true` or `false` depending on whether it loaded ok). Hope it helps!