4.0.0

• Add support for refs!
• Add support for defaultProps (static property on Components)
• Possibly Breaking Change: cache rendered DOM with components when recycling
• Possibly Breaking Change: don't wipe recycled DOM nodes, simply diff when re-used
• Use setImmediate() to improve setState() performance where available
• Skip empty values when serializing style objects
• Coerce falsey className and style values to an empty string to avoid class="null" and style="null"
• Rename missing component insert from <x-undefined-element> to <undefined>
• Use a Symbol for storing the prop cache where available