enforce existence for classes

Currently, enforce-existence only enforces the existence of jsdoc comments for functions, but not for classes. I.e. in the following `constructor` is required to have a jsdoc comment, but the class itself is not.

``` js
class Point {
  constructor(x, y) {
    this.x = x;
    this.y = y;
  }
}
```

I'd like it, if the rule was also enforce the existence of jsdoc comments on classes.

Thoughts?
