Connection mode command

This one is for Aimee's request, #12 and has a dependency on pending pull #15

pantheon-site-connection-mode / psite-cmode

The command wraps terminus_api_site_environment_onserverdev_set/get.

Example usage:
```
$ drush psite-cmode mysite dev
Connection mode: Git                                                                     [ok]
$ drush psite-cmode mysite dev sftp
Connection mode set to: SFTP                                                             [ok]
```

Command name and args are based on what you see in the Pantheon UI.