[CreateProject] allowed pretty constraint

This PR allows to put pretty constrains when creating a project. It would make instructions easier for frameworks installable through composer, for example Symfony says : 
``` bash
$ php composer.phar create-project symfony/framework-standard-edition /path/to/webroot/Symfony 2.3.0
```
"For an exact version, replace "2.3.0" with the latest Symfony version. For details, see the Symfony Installation Page"
http://symfony.com/doc/current/book/installation.html#option-1-composer

With this patch the version could be changed to "2.3.*" or "~2.3" for example, what do you think?