Remove "npm install loopback-explorer" log

loopback-explorer is a devDependency that is not included when
the project is installed as a dependency.

This commit changes both the explorer boot script template and `app.js`
to print the message only when the server was started via `app.listen`.
Merge pull request #168 from strongloop/fix/explorer-message

Remove "npm install loopback-explorer" log
v3.5.2
Merge branch 'release/3.5.2' into production
Merge tag 'v3.5.2'

3.5.2

 * Remove "npm install loopback-explorer" log (Miroslav Bajtoš)

 * ModelDefinition: make idInjection true by default (Miroslav Bajtoš)
Add remoting options to server/config.json
Merge pull request #164 from strongloop/feature/add-remoting-options

Add remoting options to server/config.json
template/api-server: introduce middleware.json

Rework the scaffolded server app to register middleware via
the new `middleware.json` file.

Note: `serve-static` is kept inside `server/server.js` for now,
as there isn't any mechanism for resolving paths in middleware config
specified in `middleware.json`.