Upgraded spritesmith version to 0.19.2

Added pngsmith as an engine, it make spritesmith work without phantomjs, gm , canvas engine.
phantomjs, gm , canvas engine have to install manual, but pngsmith engine will be installed by "npm install"
 
https://github.com/Ensighten/spritesmith/commit/215c11b23f084128e4865d610e93fad38fb546e0