Add multicast operator.
Add multicast operator.

Summary: Spring now ensures that its valueStream is multicasted so that subsequent subscriptions to the same spring will no longer create a brand new spring with the same reactive properties.

Reviewers: chuga, O4 Material Apple platform reviewers, O2 Material Motion, #material_motion

Reviewed By: chuga, O4 Material Apple platform reviewers

Tags: #material_motion

Differential Revision: http://codereview.cc/D2485

material-motion/material-motion-swift,material-motion/material-motion-swift