Update iOS sample app to use non-deprecated `MR_createEntityInContext:`
Remove missing Mac sample application
Ensure that by default, our saving methods target the root saving context on a background thread, rather than the default context on the main thread.

Addresses #735.
Label deprecated methods in header with a "DO NOT USE" pragma warning
Set default names for internal temporary saving contexts, and improve logging
Make sample app log verbosely for demonstration purposes