update readme
move pdf handling into its own module
function to split multipage pdf
use sips to convert multipage pdf to list of pngs
change .preview to return a list of pngs
change test for new preview api
add tests for artboards
update the read me with sketch.pages and sketch.artboards usage info
change read me title
add download link for Xcode command line tools
only use 2.7 runtime
fix broken art board test
add test for graffle file saved as package
add test for plain text file
Merge pull request #4 from Pixelapse/feature/png

Feature/png