Upgraded to match Ember CLI 1.5.0 with contentFor options.

This PR enables the addon to make use of the new `{{content-for}}` hooks introduced in Ember CLI 1.4.0. Specifically:

```hbs
{{content-for 'head-footer'}}
{{!-- And --}}
{{content-for 'body-footer'}}
```

This is done through the addition of two new options that can be customizes in your app's Brocfile:

```js
// Ember CLI less than v1.4.0
var app = new EmberApp({
  emberCliConcat: {
    scriptsContentFor: 'body-footer'
    stylesContentFor: 'head-footer'
  }
});
```

Apps using Ember CLI 1.3 or lower will have to change these default values:

```js
// Ember CLI less than v1.4.0
var app = new EmberApp({
  emberCliConcat: {
    scriptsContentFor: 'body'
    stylesContentFor: 'head'
  }
});
```