call initialize() from Obscura constructor

adding initialize() method would be nice to have if you want to extend Backbone.Obscura

Thanks!
Since Obscura inherits from `Backbone.Collection` ([link](https://github.com/jmorrell/backbone.obscura/blob/master/index.js#L128)), we already have backbone's noop `initialize` in the prototype chain, so adding it again isn't strictly necessary.