Updated artisan command line tool to be executable from anywhere

The script now detects from where it's run. 
the artisan tool can be somewhere in your PATH and you can manage any of your laravel 4 installs.
You just need to be somewhere inside the framework's folder.

Usage : 
```bash
cd /path/to/laravel
cp artisan /usr/local/bin/artisan

# all these will work
artisan list

cd /path/to/laravel/app/models
artisan list

cd /path/to/laravel/vendor/laravel
artisan list
```