Add new implementation that matches YUI's soon

This is a reimplementation of the original module based on the new YUI
core implementation. It uses 3 options: MutationObserver, MessageChannel
and regular postMessage. There is no readystatechange version.
Make jslint happy by creating functions outside the loop
Fix MessageChannel implementation
Update Shifter to avoid issues in older versions
Use relative path to module in tests
Merge pull request #1 from juandopazo/new-impl

New implementation that matches YUI's `soon`