fix: forward static functions in babel

[ktsn]

Static methods are defined as non-enumerable properties in Babel, so Babel users cannot use static methods.

I've changed the implementation with getOwnPropertyNames and use the original property descriptor on defining the property on the extended constructor.

This also changes the extended constructor's name and length property. But it probably does not affect user's code.

fix #186

[HerringtonDarkholme]

IIRC some environment, say PhantomJS, disallow reassigning name or length. This might be a pitfall.

[ktsn]

It looks disallow reassigning but configurable, so we can use defineProperty for it.
I'm not sure whether all browsers do not have unconfigurable properties, though.