Create a gctl register sub-command

Create a `$ gctl register` command that will register a new user account on a gasper master instance.

## Expected use
```
$  gasper register -m <url-of-master-api>
Email: <your-email>
Username: <your-name>
Password: <your-password> # masked
Re-type Password: <your-password>
```

Returns `New User Created` if the email was not pre-registered or `User Already Exists` if email is registered.