Convenience Functions

In my current project with Urho3D, I'm finding the need to use certain functions related to node orientation and positioning. To facilitate easier use of such operations, I propose the following set of additions:

- A method for the quaternion class to allow for [normalized linear interpolation](http://keithmaggio.wordpress.com/2011/02/15/math-magician-lerp-slerp-and-nlerp/) as a cheaper means for interpolation quaternions.
- A quaternion utility class to be constructed to represent a "look toward" rotation, with the input parameters being the desired direction and the desired up vector for the rotation.
- Methods for the Node class to retrieve the parent- and world-space right and up vectors in a similar manner to the `Node::GetDirection()` and `Node::GetWorldDirection()` functions.