The basic approach is to introduce a new elt.data() called $classCounts that keeps
track of how many times ngClass, ngClassEven, or ngClassOdd tries to add a given class.
The class is added only when the count goes from 0 to 1, and removed only when the
count hits 0.
To avoid duplicating work, some of the logic for checking which classes
to add/remove move into this directive and the directive calls $animate.
Closes#5271
Brian Ford
55fe6d6331