See #197. Potential fix, for discussion. Drop setImmediate and MessageChannel in favor of MutationObserver.  This forces falling back to setTimeout in IE < 11, and Safari < 6 (see https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver#Browser_compatibility).
Remove MO unload workaround.  If we are presented with real evidence that it is still a problem, we can add it back easily enough.  The webkit ticket appears to have been closed quite a while ago: Jan 2013.
Merge pull request #198 from cujojs/mutation-observer

Switch to MutationObserver to workaround IE10 setImmediate/MessageChannel/postMessage bug