Mix in support for Backbone.Collection and helper methods

It would be nice when using a component with a `Backbone.Collection` to be able to pass the collection in under the `collection` key and access it using `this.getCollection`.

* Mix `React.BackboneMixin` into the component with support for `Backbone.Collection`
* Add `getCollection` and `collection` methods to `React.BackboneViewMixin`
* Use consistent indentation throughout `README.md`