Added feature to be able to provide the api key as an environment variable

Useful for running "dotcloud setup" in an unattended way:

```console
(dotcloud-cli)pablo@eulogia:~/pch/dotcloud-cli$ export DOTCLOUD_API_KEY="N...8n:60...9519"
(dotcloud-cli)pablo@eulogia:~/pch/dotcloud-cli$ dotcloud setup --api-key
==> Using API key configured at environment variable DOTCLOUD_API_KEY.
==> dotCloud authentication is complete! You are recommended to run `dotcloud check` now.
(dotcloud-cli)pablo@eulogia:~/pch/dotcloud-cli$ dotcloud check
==> Checking the authentication status
==> Client is authenticated as andresriancho
(dotcloud-cli)pablo@eulogia:~/pch/dotcloud-cli$
```