Deleting programs

Adds the concept of a default program, and a management command to make this happen in existing data.

Adds the ability to delete a program.

http://manage.dimagi.com/default.asp?134960
i see - you're already getting all the json docs back above, so this is a bit silly. so i think either make the change above or just call `Product.by_program_id(wrap=True)` here. not sure we ever expect orders of magnitude more than hundreds of products in a program so not using `iter_docs` is likely ok