Limit transform interpolation to transition.attr.

D3’s transform interpolator is designed for SVG transforms; unlike CSS3 transforms, SVG transforms do not have units and are limited to 2D. This commit limits transform interpolation to transition.attr, so that it only applies to SVG transforms and not CSS3 transforms which are assigned via style. Thus, if transition.style is used to interpolate the transform style, the default string interpolator will now be used.

Note that string interpolation of CSS3 transforms is still likely to break; you should use CSS3 transitions instead. Fixes #1323.