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.

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 (see ngModel for
inspiration).

Fixes #747.