Added originPoint property to Body

The patch adds ability to change origin point of rotation. Along with `angularVelocity` it should be very useful.
The point can be defined in two ways:
- using QML property `Item.transformOrigin`. Actually it's a little limited. You can define only 9 points (center, corners etc.)
- using new `originPoint` property. A point is defines in local coordinates of `Body`.

Conversion from `transformOrigin` to `originPoint` executed automatically (but not back)

Known issues:
- when you do the following:
  * transformOrigin = Item.Center
  * originPoint = Qt.point(0,0)
  * transformOrigin = Item.Center
  
the last action will be ignored just because transformOrigin was not changed from QML view

As usual, added a simple example to test the functionality.