Closed over global TAFFY variable.

When using external js libraries in your own project, it is useful to move any global variables those libraries create into your own namespace. Doing this with TaffyDB causes TaffyDB to no longer work, since it relies on there being a global TAFFY variable. This fix closes over that variable, so that even if you move TAFFY to, say, MyApp.TAFFY, TaffyDB will continue to work.

All tests pass.