Wrap pgbackups:transfer in confirm

Wrap the actual transfer in pgbackups:transfer in a confirm block, because it's
destructive.

alternative to #60
maybe a typo in this method?
fix typo in resolve

thanks @crakalakin
Add WARNING
Merge branch 'pg_transfer_confirm'

* pg_transfer_confirm:
  Add WARNING
  fix typo in resolve
  Wrap pgbackups:transfer in confirm

Conflicts:
	lib/heroku/command/pgbackups.rb