UI tabs

Getting the ball rolling here with a polymer ui-tabs element. I found it easier to start with Polymer but so far I don't see anything that would make it super prohibitive to convert it to vanilla web components.

The biggest insight I gained from this was primarily around the `options` object and the `active` option. `options.active` is typically a number or a boolean, whereas `this.active` is usually a jQuery object. I got them a little confused at first but managed to sort it out. Because I'm not using an `options` object, I used `this.activeElement` to differentiate between `options.active` and `this.active`. It might make more sense to bring the `options` object back as a place to hold initial configuration data.