Warp Perspective Transform

Adds entries to Matrix.cc for getPerspectiveTransform and warpPerspective; additionally adds an example warp_image.js.

the warpPerspective call is structured much like every other matrix transformation wrapper.  the getPerspectiveTransform is a bit of an odd duck, it doesn't operate on an image but just on a source and target set of corners.  Because it is done in support of warpPerspective, i've added it to matrix.cc, but perhaps it more properly belongs elsewhere.  Feedback welcome.