Execute onbegin and onend scripts after animation begin/end

The onbegin, onend and onrepeat attributes are specified at
http://www.w3.org/TR/SVG/script.html#AnimationEvents

The scripts are executed asynchronously some time after animations begin or end, using the same timing as events.

Note: repeat events have not yet been implemented.
Merge pull request #62 from ewilligers/onbegin

Execute onbegin and onend scripts after animation begin/end