* add transform-class-properties to stage 3, set spec mode to default
* update readme with examples; use `buildUndefinedNode()`; change behavior to always define both static and nonstatic class properties regardless of spec/loose mode; update tests
static staticProperty, defined in line 18, value is 'babeliscool' but comment on line 33 the given sample output is 'babelIsCool'.
this commit fixes this inconsistency
* Desugar class properties to Object.defineProperty per spec
* Define uninitialized static class properties with value=undefined
* Make class-properties increased spec compliance opt-in (spec: true)
