Add reactive UILabel type with text property.
Add reactive UILabel type with text property.

Summary: Closes https://github.com/material-motion/material-motion-swift/issues/84

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

Reviewed By: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, markwei

Tags: #material_motion

Differential Revision: http://codereview.cc/D3209

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