Add box.json, support for generating PHAR file

This PR adds a `box.json` file, that can be used with [kherge/Box](https://github.com/kherge/Box) to generate a `phar` file for Athletic. 

Instructions:

1. Install [kherge/Box](https://github.com/kherge/Box)
2. Assuming `box.phar` is somewhere in your `$PATH`, run:
```shell
$ cd athletic/
$ box.phar build
```
3. `athletic.phar` will be present in the Athletic installation dir. This file was also added to `.gitignore`

Cheers!