Update README.md

Preview added
Switching to TAU at @TrevorSayre's request.
determine drawing function in .add() and .set()
Merge pull request #5 from gasperk/master

determine drawing function in .add() and .set()
Merge branch 'master' of https://github.com/darkskyapp/skycons into darkskyapp-master

Conflicts:
	skycons.js
Merge branch 'darkskyapp-master'
remove disabled code
fix merge mistake
demo link
demo link
demo link
Merge branch 'master' into gh-pages