app transfer commands

```
➜ hk -a testtransferapp transfer raulb@heroku.com

➜ hk -a testtransferapp transfer-decline

➜ hk -a testtransferapp transfer-accept

➜ hk transfers -l                                
testtransferapp  raulb@her…  b@heroku.…  accepted  Dec  5 15:24
testtransferapp  raulb@her…  b@heroku.…  declined  Dec  5 15:23
testtransferapp  b@heroku.…  raulb@her…  pending   Dec  5 15:29
testtransferapp  b@heroku.…  raulb@her…  accepted  Dec  5 15:22

➜ hk -a testtransferapp transfer-cancel

➜ hk transfers -l                      
testtransferapp  raulb@her…  b@heroku.…  declined  Dec  5 15:23
testtransferapp  b@heroku.…  raulb@her…  pending   Dec  5 15:29
testtransferapp  b@heroku.…  raulb@her…  accepted  Dec  5 15:22
```

IMO, it's starting to become clear that these state-changing commands should have some sort of confirmation as output. But that's a separate issue we'll have to tackle.

/cc @raulb @max