Fixes a voodoo problem with rounding in ExtrudeGeometry

### Problem
The function getBevelVec() in THREE.ExtrudeGeometry.addShape suffers from problems with rounding, preventing the proper detection of parallel lines. This can lead to "almost" devision by zero.
The effects can be seen in http://threejs.org/examples/#webgl_geometry_text :
* type in "1" and look at the back of it.
* type in "LM" the L appears but when typing the "M", the whole text vanishes. A "M" alone shows properly.

### Solution

Complete rewrite of the function getBevelVec() in THREE.ExtrudeGeometry.addShape .

Unfortunately there might be several similar rounding problems hidden anywhere in THREE.js .
Here it was testing for equality after normalizing (which uses division and square root).