bootstrap-sass compatibility

As a reply to #203.
This works fine in my project, but I could miss smth.
This solution uses compass assetic filter to do magic

My composer.json:
```json
    "repositories": [
        {
            "type": "vcs",
            "url": "https://github.com/sandello-alkr/bootstrap-bundle"
        }
    ],
    "require": {
...
        "braincrafted/bootstrap-bundle": "dev-sass",
        "twbs/bootstrap-sass": "3.1.*",
...
    },
```

config.yml:

```yml
assetic:
    debug:          %kernel.debug%
    use_controller: false
    filters:
        compass: 
            bin: /usr/local/bin/compass
        cssrewrite: ~
        uglifyjs2: ~

braincrafted_bootstrap:
    output_dir:
    assets_dir: %kernel.root_dir%/../vendor/twbs/bootstrap-sass/vendor/assets
    jquery_path: %kernel.root_dir%/../vendor/jquery/jquery/jquery-1.10.2.js
    less_filter: sass # "less", "lessphp" or "none"
    auto_configure:
        assetic: true
        twig: false
        knp_menu: true
        knp_paginator: true
```

In templates:

```html
{% stylesheets filter="compass"
        "@BraincraftedBootstrapBundle/Resources/sass/*"
        "@CMSBundle/Resources/assets/css/*"
    %}
        <link rel="stylesheet" href="{{ asset_url }}" />
{% endstylesheets %}
```
instead of 

```html
<link href="{{ asset('css/bootstrap.css') }}" rel="stylesheet" media="screen">
```

To customize variables or any other part I recomment to use <a href="http://symfony.com/doc/current/cookbook/bundles/inheritance.html">bundle inheritance</a>.
If needed I can provide deeper explanation. Questions and offers are welcome.