Add ES6 String.prototype.startsWith polyfill

[markwaddle]

Older JS engines, such as the one with Node v0.12.2, throw an exception that .startsWith is undefined. Including this polyfill fixes the problem.

I opted to include this minimal polyfill instead of the full babel-polyfill in an attempt to keep in line with the minimalist style of this project.

This is a follow-up to the failed PR #23

[markwaddle]

Oddly the require with string.prototype.startsWith worked with npm test locally. Hopefully this change will fix it in CI.