Add composer file

It would be great to have composer.json and official packagist package (twitter and fontawesome do that already) in the main repository so selectize would be installable as composer (current PHP package manager) package.

Edit: At the moment, we have our own maintained package repository: https://packagist.org/packages/brianreavis/selectize.js as we need your library as a package. Will be removed whenever you consider adding composer.json file.