Add support for tweening values relatively. #72

When creating a new Tween instance, all object proprieties is now stored
on '_valuesStart' instance variable, for possible further increment, by
the relative tweening behaviour.

When using 'repeat' method, the start value will be updated at the end
of each animation.

I've also updated "Tween non-existing property" and "Tween null property" tests, which in my perspective, they should always run, even on these cases.

Live example: http://jsfiddle.net/endel/QjXCc/2/