Added box.json for phar packaging with http://box-project.org/ 

http://box-project.org/ greatly simplify phar creation. I found it really useful especially for command line tools like phinx. 

The box.json files defines how the library should be packaged.

Packaging is as simple as

```bash
git clone git://github.com/robmorgan/phinx.git
cd phinx
composer install
php /path/to/box.phar build
```