feat(tabs): adds default transitions for tab content

By default, now tab content will animate in from whichever direction the
ink-bar is moving.  These animations can be overridden through the
following classes:

When a tab is selected, the following classes are added in order:
1. `md-transition-rtl`: flags the animation as right-to-left
2. `ng-animate`: added at the start of all animations
3. `ng-hide-remove`
4. `ng-hide-remove-active`

When a tab is deselected, the following classes are added in order:
1. `md-transition-rtl`: flags the animation as right-to-left
2. `ng-animate`: added at the start of all animations
3. `ng-hide-add`
4. `ng-hide-add-active`

Closes #717.
Closes #811.
That shouldn't impact anything since it's automatically evaluated as boolean; however, the `isDefined` check is no longer necessary, and I will remove that.

I'm also looking into ways to factor out the passing of `rightToLeft` as @ajoslin suggested, but it is looking like any worthwhile change will end up being fairly time-consuming.