Cleaned up embedded shell and added cleanup method to InteractiveShell. * Added explicit code in InteractiveShell to make sure it cleans itself up. This is now done by the single method .cleanup, that can be called to have InteractiveShell cleanup. We can't use __del__ because of the many cycles in our object graph. * The embedded shell is refactored to no embedding logic is in the base class. Thus, InteractiveShell no longer takes an ``embedded`` argument. Just use InteractiveShellEmbed. * Created a super simple top-level :func:`embed` function that creates an InteractiveShellEmbed and calls it.