Use file move command that can work across devices

I'm running Linux, and when running lad, if the location for the new project is on a different partition to the partition that /tmp is on, the File.rename command fails.

```
Moving project files                  XX
/var/lib/gems/1.9.1/gems/lad-0.1.2/lib/lad.rb:94:in `rename': Invalid cross-device link - (/tmp/MyNewProject, /home/foo/MyNewProject) (Errno::EXDEV)
        from /var/lib/gems/1.9.1/gems/lad-0.1.2/lib/lad.rb:94:in `block in execute'
        from /var/lib/gems/1.9.1/gems/lad-0.1.2/lib/console.rb:22:in `call'
        from /var/lib/gems/1.9.1/gems/lad-0.1.2/lib/console.rb:22:in `task'
        from /var/lib/gems/1.9.1/gems/lad-0.1.2/lib/lad.rb:93:in `execute'
        from /var/lib/gems/1.9.1/gems/lad-0.1.2/bin/lad:15:in `<top (required)>'
        from /usr/local/bin/lad:23:in `load'
        from /usr/local/bin/lad:23:in `<main>'
```

This is resolved if mv() is used rather than File.rename.