Update AMD define to export Howler and Howl as single module.

It is recommended to only define one module per file. See http://requirejs.org/docs/api.html#define

Currently there are two modules defined, and Howl requires Howler to work. So as currently implemented you would always have to include both modules. I've made an update that will export both Howler and Howl as a single AMD module.

Also, when you load Howler as an AMD module, ideally the Howler and Howl are no longer in the global namespace.