JS Hook

`data-js="login"` vs. the old `class="js-login"`. See commit for reasons.
Sure. I know data lookups are slightly slower, but really, only marginally so since we're scoping to the element anyway with `this.$()`.

Messier in some ways, but cleaner in others. It reduces the messiness of `class=""`, and makes JS hooks easier to visually identify.

Giving a jQuery example is a good idea. Will do.