Rename client.js to slave.js

Rename the client.js script to slave.js, and changed places where it's
refered to as client into slave.

package.json

@@ -10,7 +10,6 @@
     "cover": "nyc npm test",
     "fast-cover": "FAST_TEST=y nyc npm test",
     "ci-cover": "nyc --reporter=lcovonly npm run-script test",
-    "start": "echo Run node client start [instance] to start slaves && node master",
     "install": "node lib/npmPostinstall",
     "bundle-dependencies": "bundle-dependencies"
   },