Add a quaternion implementation

[adbrown85]

We need a quaternion implementation to support smooth rotations.

The core methods we will need right away are:

• rotate(Vec3, angle)
• fromAxisAngle(Vec3, float)
• toMat3()
• toMat4()

We may want to make rotate a top-level function though.

In the future it would also be nice to have:

• fromMat3(Mat3)
• fromMat4(Mat4)

[adbrown85]

The rotate method should probably just be rotate(Quaternion) instead of rotate(Vec3, float).

[adbrown85]

Not sure if the default constructor should be setting the scalar component to zero or not, since a quaternion doesn't necessarily have to be a rotation. Which also brings up whether there should really be a separate class for rotations. It could be called either Rotation or maybe Versor. It would keep the components normalized, which also means keeping them private. This might be a little overkill though.

[adbrown85]

If Quaternion isn't necessarily a rotation, we should probably replace rotated with operator* and operator*=. That also makes it a little easier to specify the order of the operation, although it's backwards from what one might expect so that might be a little confusing.

[adbrown85]

Also want magnitude, normalize, and conjugate.