Define properties default value as null or undefined

[cherifGsoul]

This fix adds the ability to define properties with null or undefined default value:

class Foo extends mixinObject() {
    static get props() {
          return {
	       nullProp: { default: null },
	       undefinedProp: { default: undefined }
	 };
    }
}

var foo = new Foo();
foo.nullProp; // -> null
foo.undefinedProp; // -> undefined

Closes #148

[phillipskevin]

We also want to support

nullProp: null,
undefinedProp: undefined

which still throws with your change.