Requesting to add the constant Tau ( = 2*pi) to src/constants.js

Hello,

In honor of Tau day (today!), I've added tau to the constants.js file. 

In case you aren't familiar with this, here's the explanation from the originator, I think it's quite compelling:

http://tauday.com/tau-manifesto

Thank you for your consideration, and happy Tau day! 

(Also, this is my first public pull request, I wasn't sure which branch to pull request to).

Sincerely,
Zak Zibrat
zakzibrat@gmail.com