Commit 9e2103f9 added support for zero-length tails by conditionally
creating the line object. However it kept using that object
unconditionally in some places (such as setLength), which results in
undefined references if an ArrowHelper is initially created with a
zero-size tail and then made longer by calling setLength().

This patch fixes this problem by always creating the tail object and
limiting its minimum length to 0. This also simplifies the code a bit.

Fixes #5570

change hex to color.

changed hex to color

add THREE.Rotation and remove Object3D.useQuaternion (as we always use quaternion now.)  Following recommendation discussed here: https://github.com/mrdoob/three.js/issues/3404#issuecomment-17262516

fix ArrowHelper so that it can deal with normals pointing down [0,-1,0] via creating a unit test (makeBasis).

(Phewww!) I'm sure there are still things I missed...

Renamed Matrix4's setTranslation, setRotationX, setRotationY, setRotationZ, setRotationAxis and setScale to makeTranslation, makeRotationX, makeRotationY, makeRotationZ, makeRotationAxis and makeScale. Fixes #1615.