Added some missing mapping functions to Body

* Body.getLocalCenter()
* Body.toWorldPoint(localPoint)
* Body.toWorldVector(localVector)
* Body.toLocalPoint(worldPoint)
* Body.toLocalVector(worldVector)

For some of these of course also Item.mapToItem and Item.mapFromItem
could be used, but I don't think it hurts to have them anyway.
Merge pull request #61 from bjorn/body-functions

Added some missing mapping functions to Body