Enhance CTM example

- Inject script using `appendChild` instead of a `document.write`
- Wrap text on 80 chars
- Put CTM test in a variable
- Swap the `core` class really early (before CSS is in) to avoid any FOUC
- Shorten the class replacement regex
- Remove unnecessary `type='text/css'` and `type=`text/javascript'`
- Add link to polyfill service documentation
Refactoring async and defer attributes into booleans

Thanks to @pornel for pointing out that these attributes were booleans
Wrap string in single quotes
Merge pull request #267 from Financial-Times/ctm-script

Improve CTM example