feat(Input): enable refs on Input components

[eddywashere]

PR enables refs on Input components. I think it’s fairly common to want to add a ref to an input for doing blur/focus or other events on the dom element.

Info can be found here: https://facebook.github.io/react/docs/more-about-refs.html#cautions

Refs may not be attached to a stateless function, because the component does not have a backing instance. You can always wrap a stateless component in a standard composite component and attach a ref to the composite component.

Related to #172