adds composer.json

[mallowcat]

Adding a composer.json to the repo will allow this library to get effortlessly included into plugins/repos that are built using composer.

This is the minimal info needed and really all that's needed, but there is a plethora of additional information that can be provided if desired, an example can be seen here: http://composer.json.jolicode.com/

[jtsternberg]

Thanks!