bin/cli: rename Studio to Arc

Replace all references of Studio with Arc.

Remove the requirement to run Arc in a LoopBack application,
Arc supports non-loopback apps too.
Remove "npm install loopback-explorer" log

loopback-explorer is a devDependency that is not included when
`strong-arc` is installed globally, for example via

    npm install -g strongloop

This commit changes the explorer boot script to print the message
only when the server was started via `app.listen`.
Merge pull request #552 from strongloop/fix/help-messages

Fix help messages