Fail if alternate emails are not specified as array in config.yaml

If alternate emails for an account is not specified as an array the account initializer will fail. This now fails with a more descriptive error message. To reproduce, change in config.yaml:
```
:alternates:
  - alt1@asdf.com
```

to
```
:alternates: alt1@asdf.com
```

still failing without any good reason:
```
:alternates: - alt1@asdf.com
```
^^^ probably invalid YAML.

in response to: http://rubyforge.org/pipermail/sup-talk/2013-August/004984.html