perf(ngForm,ngModel): move initial addClass to the compile phase

Other then moving from link to compile time the only difference (which unit tests didn't seem to care about) is that `$animate.addClass(VALID_CLASS)` and `$animate.removeClass(INVALID_CLASS)` are no longer called at link time since VALID_CLASS is added at compile and INVALID_CLASS shouldn't exist. If anyone depends on the initial animation calls or on the `.removeClass(INVALID_CLASS)` then this change won't work, at least in its current form...
do `addClass(PRISTINE_CLASS).addClass(VALID_CLASS)` in order to avoid extra string concat and splitting