iOS: Fix memory leak & regression in issue #6

This also fix the regression caused by the commit b8c6f32acf that closes 
issue #6. The regression is showed as `sync` method no longer receive
the callback messages.

The solution involves using CFRetain & CFRelease to release the plugin's `self` object. It's also important to set syncHandler & scanHandler as
`nil` otherwise it will cause a memory leak.