Added EmberPrecompile filter.

Ability to use ember-precompile to precompile handlebar templates for usage with Ember.js.

Tested with stable release of Ember.js 1.2.0, Handlebars.js 1.2.1

Example configuration:
```
assetic:
    ....
    filters:
        cssrewrite: ~
        emberprecompile:
            bin: /usr/local/bin/ember-precompile
            apply_to: "\.hbs$"
```

```
            {% javascripts filter='?emberprecompile' output='js/compiled-handlebars.js'
            '@YourBundle/Resources/public/hbs/application.hbs'
            '@YourBundle/Resources/public/hbs/index.hbs'
            %}
            <script type="application/javascript" src="{{ asset_url }}"></script>
            {% endjavascripts %}
```
Install by using the following command-line:
```
npm install ember-precompile -g
```