Allow to call getter/setter logic When subclassing

http://typescript.codeplex.com/workitem/645

At the moment it's not possible to call super getter/setter when overriding properties:

class A {
    private _property: string;
    get property() { return this._property; }
    set property(value: string) { this._property = value }
}

class B extends A {
    set property(value: string) {
        super.property = value + " addition";
    }
}