Add a rotation property to RegularPolygon

[bfoz]

Generating the vertices requires knowing the angle of the first vertex, which would be equal to the rotation property. It currently assumes that the first vertex is on the X-axis, which is fine for most cases, but doesn't work when sides == 4 and isn't ideal for pentagons. Alternatively, special case those two situations.

[bfoz]

Alternatively, the rotation could be specified using an axis passing through the center of the polygon (using either a direction vector, or a point)