Crafty.math.Vector2D improvements

[mucaho]

this PR was split from #708

• crossProduct method added
• adds optional argument to save the result into instead of instantiating a new object
• various minor test fixes

[starwed]

This looks good to me, although I honestly have no idea what the (existing) getNorm function would be used for? (Also, it doesn't normalize the resultant vector in one of the use cases, which seems suspicious.)

Anyway, that's a problem with the existing code, so I'll go ahead and merge this.

[mucaho]

getNormal() returns two different values (depending on whether an optional parameter was filled or not):

• In the trivial case it returns the perpendicular vector to this vector (maybe add documentation for that?)
• As you mentioned in your other post, the second return value represents an unit vector perpendicular to the line between two vectors