use scale instead of overwriting the matrix each time the current changes

[zepumph]

It seems wrong to do have this line in a link function in RightAngleArrow.js:

`  self.matrix = Matrix3.scale( scale );`

When I converted RightAngleArrow.js to inherit from Path, this line ruined the transform unless I added a mutate call under it:

      self.matrix = Matrix3.scale( scale );
      self.mutate( options );

I don't yet know how I should fix it.

[zepumph]

this issue is from code review: #51

@jessegreenberg and I were able to simplify this operation nicely, closing