Allow rsync command to be silenced. Fixes #11

This doesn't eliminate the command's output completely. It does two things

1. Adds an empty array as the second argument to `this.exec()` so that `options` is actually passed along in the right (third) argument
2. Has the command runner look for `options.silent`, and if it's set, it's abbreviates the `rsync` command.

It does this because when you are `rsync`ing a huge list of files, the output is a wall of text, even when silent is set.

This doesn't eliminate the command from logging (but maybe it should.)