[added] MotionObservable.from(stream)
[added] MotionObservable.from(stream)

Summary: Closes https://github.com/material-motion/material-motion-js/issues/168

Reviewers: O3 Material JavaScript platform reviewers, O2 Material Motion, #material_motion, featherless

Reviewed By: O2 Material Motion, #material_motion, featherless

Subscribers: featherless

Tags: #material_motion

Differential Revision: http://codereview.cc/D2958

material-motion/material-motion-js,material-motion/material-motion-js,material-motion/material-motion-js