ENH: set display.max_seq_items default != None
closes #3391.

No idea what the default value should be. numpy equivalent default is 1000, but even 100 is too big IMO. 
The default is primarily there to mitigate output bombs when displaying some pandas objects.

Still need to update the release notes on all these micro-PRs.

@y-p I think this ought to go in 0.13
 I'm down with that. I'll fix the broken tests and add some docs.
 Added docs and hopefully green. Last chance to bikeshed on default value of 100.
