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.
I don't like the idea of adding this boolean everywhere in the tabs. Why don't we just watch the selectedIndex, and compare the old and new value, instead of having to add all this complexity?

Alternatively, onSelect and onDeslect could be passed a previousIndex variable.