Support 'values' and 'from', 'to' in animateMotion

Motion path animation using animateMotion can specify a sequence of
translates using values or from and to.

For concise testing, we make use of additive="sum" to combine the
effects of animateMotion elements on a given visual element.
Merge pull request #50 from ewilligers/additive-example

Support 'values' and 'from', 'to' in animateMotion