Some improvements and refactoring

I've added few improvements such as `ActionController::Base.helpers.asset_path` instead of self-writen resolver. It works using your environments so there is no more reasons to handle it with `config.assets.digest` or anything else.

I've also added option `use_file_filter` to handle how to resolve paths. For example in my projects i have `config.assets.precompile` configured only in production so these path won't be resolved in development. By default this options is set to `true`.

Also i've fixed `app_assets.js`. Now list of files is hidden under local object and all code sits under self-invoking function. This fix was made to cleanup global object. But if you need to access files list directly you can call `files` on `asset_path` function:

```javascript
var files = asset_path.files
```

And i've made total refactoring of `JsAssets::List` to make it more readable and simple.