Fix irregular Timestamp arithmetic types #6543
closes #6543

This is my first PR so I may have done a few things wrong. If I should have added my tests to a different file/location, please let me know.

I should point out that this code is also tested by many other sections of the pandas codebase, on my first pass I hadn't captured the "Timestamp - datetime" case and all sorts of things blew up. Better now. :)
