new feature: pausable animation

Hi,
Currently there is no way to pause an animation, although it could be useful (think for instance to the pause button of a game, which needs to pause every widget).

Trying to address that case using the provided stop() and start() functions defined on the Animation class is not a solution, as one does want to restart where it just as been paused, and not from the begining (specially when dealing with sequences of animation).

That's the goal of this pull request, the slight modifications I've made are:
- adds a pause property,
- fires a on_pause event,
- avoids to update the properties on the _update function.

That's it!

I'll appreciate for reviewers to peek in, and hoping this is useful enough to be merged.

Regards