new SketchTool 3.1
move old test sketch files to v3.0 dir
add v3.1 sketch test files
test against sketch 3.0 and 3.1 formats
add progress bar to tests; tests for art boards: slash
ensure sketch migrate is executable
recursively check for preview output
Merge pull request #9 from Pixelapse/feature/sketch-3.1

Support for Sketch 3.1 File format