[refactored] lowerBound and upperBound into single clampTo operator
Continuation of https://github.com/material-motion/material-motion-js/issues/193
Since we are using named args now, it doesn't make sense to have separate upperBound and lowerBound operators. The combined operator uses _mathOperator under the hood, so it works on points as well as numbers.
Reviewed By: Material Motion, vietanh
Tags: Material Motion
Differential Revision: http://codereview.cc/D3405