fix(tabs): Initial tab selection

- Avoid re-initializing `tab.active`
- `setActive` only when all `tab.active` are set up (on the next
  digest cycle)

Before I go to explain, there are up to two expressions that indicate
whether a tab is active:

  1. `active` variable in the isolate scope of the tab directive
  2. The expression from the active attribute (`attrs.active`) if it is set, I'll call this `getActive`, as that's the variable that refers to it (`setActive` assigns to it).

During initial linking (adding of tabs), the `active` variable in the tab's isolate scope tracks the active tab. When the first tab is added, it's `active` is set to true since there's no way to know if subsequent tabs are active since they haven't been added yet. As such, at this point, it is not meaningful to set assign the `getActive` with the value of `active`. At least not until all the tabs have been added. Hence, a good time would be to wait until the next $digest cycle.

A watcher is called asynchronously after initialization even if there is no change on the expression's value.

As such, we can leave that to the watcher for the `active` expression to initialize getActive by calling setActive during its initlization cycle.

However, there is a chance (if the $digest cycles and planets align...) that the `active` variable gets initialized a second time using the `getActive` (in the watcher for `getActive`). Since we're already setting `active` to `getActive`, and the `active` variable should now be carrying the *truth*. Avoid this re-initialization.

---

This fix is ugly because the logic for the two-way binding of `active` and the `getActive` is complex. Consider refactoring the implementation for maintaining the two-way binding. It looks like this sort of binding is exactly what `$compile` already does in [compile.js#L973-L1001](https://github.com/angular/angular.js/blob/c173ca412878d537b18df01f39e400ea48a4b398/src/ng/compile.js#L973-L1001) . AngularJS 1.1.4 introduced the feature in https://github.com/angular/angular.js/commit/ac899d0da59157fa1c6429510791b6c3103d9401 to allow optional binding of variables in the isolate scope that handles this very cleanly just by declaring `scope: {active: '=? active'}` for the directive.

I didn't investigate into why the previous test with `ng-repeat` didn't expose this problem. It's possible it's because it had an additional digest cycle between linking such that the digest cycles didn't go in the order that would expose the problem.

Fixes #747.