[WIP] Add support for different bootstrap version

The different versions of bootstrap are not loaded the same way. For example the `vendor/` directory in `bootstrap-sass` has been removed in bootstrap 3.2. This has lead to some PR (#922, #912, #929, #939) that introduce BC break.

The solution introduce by this PR is to use the config.yml to setup the assets. Each required change in assets is then reported in a specific config file. So we just have to import the desired config file according to the bootstrap version : 

```yml
imports:
    - { resource: "@MopaBootstrapBundle/Resources/config/assetic/bootstrap_3.2_sass.yml" }
```