Add more validations of user input and instructions to cd <app-dir>

/to @bajtos 
/cc @altsang 
/cc @cgole 

The PR includes the following fixes:
- adds a conditional message if the app dir is not `.`.
- adds more validations to user input for names
- fixes the url-not-found warnings on jquery map and favicon for loopback:example

`name` is the application name, not the directory name. It may be pre-filled by the command-line arg and/or the directory name.

The directory name is processed immediately and not stored anywhere - see [lib/actions.js](https://github.com/strongloop/generator-loopback/blob/ec5b4eb1638222461c255279060a7e76aaf756c9/lib/actions.js#L27-L36). Also note that `.` is never passed as `name`.

> I just tested your use case and the patch worked as expected (no cd is printed).

Using the version 0c558aef, the example above prints `cd` step for me.