Issue #30: Added username option to SCP deployment

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

Actually, we might as well fix awkwardness up now. Just follow the following pattern to avoid the awkward if/else. :)

self.host = kwargs.pop('host')  # (Raises an error if "host" is not provided.)
self.username = kwargs.pop('username', None)
self.port = kwargs.pop('port', 22)
self.root_dir = kwargs.pop('root_dir', '')