Converter generates incorrect rotation key frames

Replaces https://github.com/KhronosGroup/glTF/issues/244

---

A COLLADA model with the following animation (hand edited at this point) has five keyframes:

``` xml


0 0.5 1.0 1.5 2.0







0 90 180 270 360







LINEAR LINEAR LINEAR LINEAR LINEAR













```

The rotation angles around Z are simply `0 90 180 270 360`. However, the converter generates the following axis-angle rotations, which include an axis flip:

```
[1, 0, 0, 0]
[0, 0, 1, 1.5707963705062866]
[0, 0, 1, 3.1415927410125732]
[0, 0, -1, 1.5707963705062866]
[0, 0, -1, 0]
```

Adding more keyframes (with shorter deltas between the rotation angles) does not fix the problem nor does changing the domain from `[0, 360]` to `[-180, 180]`.

Start by reproducing the issue with the provided COLLADA animation and trace the converter's rotation-animation handling. Done means the generated axis-angle keyframes represent the continuous 0, 90, 180, 270, and 360 degree Z rotation without an axis flip.

Written by the indexing model from the issue text.