Added 'args' and 'ignore-errors' options and fix bugs

Added 'args' option to add additional arguments to rsync (like --delete, --exclude etc.) as a space separated list.

Made source and target optional *during buildout process* when setting the 'ignore-errors' option to true. Need this to prepare defaults for doc-deployment in a master buildout. Target is the only option sub-buildouts have to set if they want to deploy any documentation.

Fixed bug where port is not specified but script is true. This led to the argument port='None' to rsync.