Install fixes

I had an issue running the usage commands where `wintersmith-node-sass` was not available.

	starting preview server
	using config file: config.json
	LiveReload listening on port 35729

	error Error loading plugin 'wintersmith-node-sass': Cannot find module 'wintersmith-node-sass'
Adding the plugin to the package.json fixed this.

`ws` is not a command wintermsith installs, I'm assuming you must have some sort of alias. I've updated the usage instructions to reference `wintersmith` instead.