Move setting of Soundcloud client_id into a configurable provider

This lets you use your own Soundcloud client ID by configuring the provider in a config block. The provider is set to use the client ID you originally had by default, so Plangular still works out of the box, but this lets you override it if you want.

**Example configuration:**

~~~
.config(function(plangularConfigProvider){
  plangularConfigProvider.clientId = '[YOUR_CLIENT_ID_HERE]';
})
~~~

And you access that value with `plangularConfig.clientId`
I often see (and do) `var self = this`. is there any reasoning behind the underscore convention? (Asking because I'm fairly new to JavaScript)