Added Matrix3[d].Add and Matrix4[d].CreateFromRotationMatrix

I was implementing the [Direction Cosine Matrix](http://www.starlino.com/dcm_tutorial.html) algorithm to calculate the orientation of a device with an IMU.

The process involves a lot of math with 3x3 rotation matrices, and I found that 2 methods I needed were missing from OpenTK, Matrix3.Add, and a way to create a Matrix4 from a rotation Matrix3, which I'm calling Matrix4.CreateFromRotationMatrix.

If you'd prefer another name for CreateFromRotationMatrix, let me know and I can add another commit to change that.

Thanks in advance!