Experimental/autosave freeze fix

This fixes the freezing issue found in [#24](https://github.com/karelia/BSManagedDocument/issues/24). Apparently `NSDocument` freezes during in-place autosave on file coordination, thus the solution is to have our own in-place autosave implementation that skips it. Since the persistent store isn't expected to move while being open anyway, it's likely safe to skip calling `NSFileCoordinator` during autosave.

I've incorporated this in my application and sent out a beta release and seems stable. Furthermore I'm making that beta live now (cross fingers )