Add Composer support

[Composer](http://getcomposer.org) is a package manager for PHP. This allows Backbone to be downloaded by putting `"documentcloud/backbone"` in your own `composer.json` file.

``` json
{
  "require": {
    "documentcloud/backbone": "0.9.*"
  }
}
```
The above will obtain both Backbone and Underscore, since Backboone requires Underscore. Thanks!