Updating PR #190 (ModelSync.Local) with Shifter

This is an update/refactor to yui/yui3#190, which has been a stale PR for a while.

It's a few bug fixes I've received from the Gallery version of ModelSync.Local since originally submitting that PR, more complete unit tests, as well as updated Model documentation.  I've also updated it to play nicely with Shifter, since the originally PR was submitted while still using the old builder.

I'll be at the roundtable tomorrow, if there's time to discuss it.  Thanks!