shim for the setImmediate efficient script yielding API

 setImmediate.js is a highly cross-browser implementation of the
 setImmediate and clearImmediate APIs, proposed by Microsoft to
 the Web Performance Working Group.
 .
 setImmediate allows scripts to yield to the browser, executing a
 given operation asynchronously, in a manner that is typically
 more efficient and consumes less power than the usual
 setTimeout(..., 0) pattern.
 .
 Node.js is an event-based server-side JavaScript engine.