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

This condition is not correct. Consider the following case:

```
$ yo loopback myapp
[?] Enter a directory name where to create the project: .
[?] What's the name of your application? myapp

(...)

Next steps:

  Change directory to your app
    $ cd myapp

(...)
```

The generators prints `cd myapp`, but there is no `myapp` directory - the app was created in the current directory.