Make scripts for the two gulp actions. Update readme to point to them.

[wbobeirne]

The current instructions to npm install then run gulp aren't quite right. That only works if you have gulp-cli installed globally already. Otherwise, you get -bash: gulp: command not found. You would have to run node_modules/.bin/gulp if you wanted to run it directly from the command line, as your $PATH isn't node_modules aware.

However, package.json scripts are node_modules aware. All this PR does is add a dev and a dist script that runs gulp and gulp clean respectively. I also updated the readme to suggest running those two instead.