Changes for React 0.12

- [x] Remove `transferPropsTo`
- [x] Remove JSX pragma
- [ ] Add example with `{...this.props}`
- [x] Note change about `defaultProps` resolution time
- [x] lowercaseComponent vs UppercaseComponent
- [x] Update to final React 0.12.0 (and jsx-loader)
- [x] Better description of `createElement`/`createFactory`?
- [x] Update wording throughout to match the React terminology doc
- [ ] Re-add JSX compilation example (with new compiled code)