Line segment math primitive class: Line3

Line3 is a line segment class.  It has a start and end.  This differs from ray which is an infinite line.

The most useful functions on Line3 at the moment are:

- ```closestPointToPoint()``` - closest point on the line to an arbitrary point.
- ```at()``` move along the line via a [0-1] parameter.

Line3 can also be used as the inputs to the other math functions that were expecting a line, thus simplifying the interface:

- ```Plane.lineIntersection( line )```
- ```Plane.isLineIntersection( line )```

We needed this for some of our internal work and I figured it would make the most sense to contribute it back.  I find it useful to centralize line related functions on Line3.