Euler, a class for storing and manipulating euler rotations

This was discussed here: https://github.com/mrdoob/three.js/issues/3404

I have implemented the basics, which is mostly just reorganization.  There are still three functions to complete but they are new functions thus they do not break existing code.

I have updated Object3D, updated the examples and updated the unit tests.  I have also maintained what I think are the appropriate contributors.

I tested all the examples I changed as well as a dozen of other examples, they all worked..

Thus the primary reorganization to accommodate the new Euler class is done.