Add support for interacting with heroku postgres built-in backup service

Hmm. Good to know, but I'm not sold on that--it calls `#to_i` on its argument and then uses integer division, meaning it loses basically all precision once you get into gigabytes. It also uses binary (kibibytes)--is that really clearer here?

In other news, this made me realize my suffixes are buggy--I skip past GB and go straight to TB. Fixing.