Issue #30: Added username option to SCP deployment

https://github.com/grow/pygrow/issues/30

Normally I'd ask you to nix the comment, and to just default `self.username` to None when initting ScpDeployment to avoid the awkward try/catch, but in the next release (in the "launchpad" branch currently), I'm redoing the config system for deployments, so I'll just clean this up there. :)