Define composer test script

The composer test script is an easy and convenient way to define your unit
testing scripts. It makes testing your library as easy as just running
`composer test`.

For more information about the `scripts` field check out [the manual](https://getcomposer.org/doc/articles/scripts.md) and [this blog post](http://maxgfeller.com/blog/2014/07/22/composer-test/).