Some fixes in Box2DFixture

* Fixed leaking of memory in case of putting vertices too close
  together, by using a QScopedArrayPointer to simplify cleanup.

* Fixed check on minimum amount of vertices for Chain fixture,
  which should be at least three for a looping chain.

* Some small coding style fixes.
Don't set previous or next vertex for looping Chain fixtures

Box2D docs say to avoid this. For a chain loop, the previous and next
vertices are automatically set as appropriate.

Also fixed the y coordinate of these vertices to be correctly inverted.
Moved vertices for Box fixture back on the stack

There is no reason why these temporarily calculated vertices should be a
member variable. It's faster to have it on the stack.
Improved Box2DBox::createShape

Use b2PolygonShape::SetAsBox, which should be a little more efficient
than setting the box as a polygon since it avoids a bunch of checks and
calculations.

Also, an initial rotation property set on a Box fixture is now
supported, though changes to it are not yet applied.
Fixed positioning support for Circle fixture

The x and y properties of the Circle fixture were being ignored
since change 272a3207fc8e71c67c3a08b3727db057bdecfab6. This change
makes it so that they are being taken into account again, while still
shifting the b2CircleShape to center in the Circle fixture item, at
least when it has its default size.

Instead of setting the width and height explicitly, the implicit
width is now set based on the radius.
Merge pull request #19 from bjorn/master

Some improvements related to fixtures