Add the API client side for backups.

Note that backups has its own facade, so it isn't quite as simple as adding a new method on Client.
It's because BackupsClient is basically a sub-client (of Client), rather than it's own stand-alone bit of the API landscape.  Rather than add all the backups-related methods directly to Client, I created this sub-client and added just it to Client.  My intent was to have that relationship reflected by what got embedded in the struct.  It's not all that important to me though and I don't mind changing it, but keep in mind that it *is* a special case (as far as I can tell).