Fix buggy ES6 polyfills

Replace the old ES6 polyfills with properly tested alternatives.

`String.fromCodePoint`: http://mths.be/fromcodepoint
`String.prototype.codePointAt`: http://mths.be/codepointat
Merge pull request #19 from mathiasbynens/fix-polyfills

Fix buggy ES6 polyfills

Thanks to Mathias Bynens for the implementation!