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.

There is a usage example on '09_relative_destination.html'.
Add tests for tweening values relatively. #72

Don't prevent interpolation of null values or of non-existing
properties. They should now always start as zero.
Merge branch 'dev-relative-destination' of git://github.com/endel/tween.js into dev
Make example 9 more JSHint-friendly O:-), also fix title to be 'relative destination', and use minified tween version
Pump revision up to 10dev. Also fix various style thingies here and there.