Inlines separated into stacked and tabular, fixes #334

Uses similair approach as the old admin. Different template is set on the inline class for tabular and stacked.

Admin2Inline can no longer be used as inline directly, therefore I also changed the import in __init__ to use tabular as a fallback to ease migration to this version.

Examples are updated to use tabular instead. Maybe one of them should use stacked, just for the sake of using it?

Not sure how to add tests for this. Admin2Inline is kind of icky to initialize. Suggestions are welcome.