ModelSync.Local implementation (final) - passing on A-grade browsers

This implements the feature discussed in #1156.

The previous problem with the PR in #385 was that there were failing tests on older browsers (IE6 and IE7) that did not support `localStorage`.  This includes the fixes for those failing tests, which now pass successfully and fallback to the internal cache.

As mentioned before, ModelSync.Local is currently being used on the [YUI TodoMVC](http://todomvc.com/architecture-examples/yui/) and has no other problems beyond the fallback in the older browsers.

Error handling is included with ModelSync.Local to handle the case when `localStorage` passes its maximum allocated space.