Warn when mixing longhand and shorthand props

[ianobermiller]

Fixes #95

[alexlande]

Nice, looks good. You've got a little lint though.

[williamboman]

Quite annoying to see these warnings when it's done deliberately, e.g.;

// utils.radium.js
export const resetBoxModel = { margin: 0, padding: 0, border: 0 };

// foo.jsx
import {resetBoxModel} from './utils.radium';

const styles = {
    base: {
        ...resetBoxModel,
        marginRight: 5,
        paddingLeft: 10,
    }
};

or are there better ways of doing this?