allowing the ability to override the credentials file such that you can ...

...specify a username per app

My overall goal by allowing a user to do this is
1) A person who has multiple accounts
2) I'm trying to automate the LDAP provisioning process in development. By doing so, I need to be able to specify development credentials in the `~/.socialcast/credentials.yml` that are specific to dev users i.e. jennifer@socialcast.com