styleBase + propTypesBase

[pedrottimark]

Follow up on #108 and supersede #115

1. styleBase(this.props, this.context) eliminates coupling to this of component for simpler testing
2. propTypesBase so components can check base style props

Suggested order to merge Component.propTypes using Object.assign:

1. propTypesBase
2. props specific to component (for example, src for Image)
3. style
4. children

TODO in slide.js review how it uses base style props, especially the apparent restriction on margin to be a single number (instead of number or string as everywhere else). Need to review progress.js too.