React 16

[Scimonster]

Make package future proof.

• Add React 16 as peer dependency
• Use prop-types package
• Use ES6 class instead of React.createClass

[Scimonster]

@benjeffery Have you seen this? It's different from #17 in that it still uses a functional initializer, so it shouldn't break anything.

[benjeffery]

Thanks for doing this and apologies for the delay in merging. I'm preparing a release now.