Add ability to send forgotten username

The `jitsu` CLI should be able to send forgotten usernames to users based on email.

Usage: 

```
   jitsu users forgot marak.squires@gmail.com
```

This should send a reminder email to marak.squires@gmail.com with the account name, or let the user know we didn't find any accounts with that email address.

This will most likely required a modification to the Nodejitsu User API. 
