Use ng-animate with ui-sref-active

I wanted to use `ng-animate` with ui-sref-active so I made a relatively simple change to the directive.

After looking at ViewDirective's use of the `$animate` service, I see that some rather non-trivial things were done for backwards compatibility, etc. I don't feel qualified to copy over that logic, or refactor that logic into a service, so you probably won't be merging my code as-is.

Still, it's here as a feature request / proof-of-concept.

I was very satisfied with the results in my project!
No. (At least sometimes)

In some more recent (back in May) versions of Angular (1.2ish IIRC), they included a stubbed `animate` service, that would pass through calls without attempting animation. If ngAnimate was included, it would override the fake `$animate` service.

However, the I am not sure if the stubbed animate is available for all the versions of angular that ui-router would like to support. Also, when I looked at the code, I noticed that ui-router already had some code in place for backwards compatibility that I wasn't ready to mess with.