Methods to enable averaging multiple transform matrices - part 2

I wrote some methods to enable weighted averaging of multiple transform matrices in order to obtain a transform which is the "weighted midway" between the given transforms. To achieve this, translations/scales are lerped and rotations are slerped. By definition, more than two Quaternions are slerped by:

```
(q_1^w_1)*(q_2^w_2)* ... *(q_n^w_n)
```

where `w_i` sum to 1. This is particularly useful when transform information from multiple observers must be combined into a single one. 

An example usage is the following, where the red cone's transform is the equal weight average of the green cones' transforms; green cones are scaled by 1, 1.5 and 2 respectively, they are not closer to the screen than one another:
![average](https://cloud.githubusercontent.com/assets/1974284/2985238/e6636a54-dc3a-11e3-95ea-70a4fab04d02.png)

Changes from part 1: Problem was related to #1859, it is taken into account now.