add library_async.js

For kripken/emscripten-fastcomp#39

Call `Browser.safeSetTimeout`, then (1) can avoid setting `noExitRuntime` yourself, and (2) it checks for an error happening meanwhile and does not do the callback if it happened. I don't think (2) should be possible here, but still it might make sense in the future, and this is the API for "safe" `setTimeout`.