Use existing backbone

@bglusman @devmanhinton 

Closes #78 

```js
// Before
var Backbone = require('backbone');

// After:
var Backbone = require('end-dash').Backbone;
```

This branch includes backbone iff there isn't already a backbone on the page. Instead of checking if the object supports the `on`, `off`, `get`, `set` interface, we check if it's a Backbone object.

It also raises two questions:

- How can we clean up dependency management? Looks pretty ugly architecturally. Can we get away from using CommonJS on the client?
- Should we include Backbone if it doesn't already exist? If we're going to say Backbone is a dependency, maybe we should just error if it isn't present. This seems a lot clearer.