GeometricPrimitive.Plane allows for separate tessellation values for X/Y axes.

Before considering this pull request, I have s few questions:

1) Are there any tests which are covering GeometricPrimitives API?

2) This is a breaking change. I quess that is okay considering Paradox Game Engine is in beta state? An alternative, which wouldn't break existing behavior, would be to create an overload instead.

The problem this pull request tries to solve is that for example when generating terrain from the plane primitive, only a single parameter is used for tessellation. This doesn't allow for rectangular grid to be made up of square cells. Currently X and Y axes can only have the same number of quads (notice the stretched floor):

![rectangular cells](http://az695587.vo.msecnd.net/images/shapes_1.PNG)

With this change the number of quads per axis can be configured separately, so for this example, I can create square cells within a rectangular grid:

![square cells](http://az695587.vo.msecnd.net/images/shapes_2.PNG)