Serve static files with combine=false and debug=true.

When using ...
````php
'debug'   => true,
'combine' => false
````

... assetic module was including individual collection files, for example:
````html
<script type="text/javascript" src="/js/main_bootstrap.min_1.js"></script>
<script type="text/javascript" src="/js/main_json2_2.js"></script>
<script type="text/javascript" src="/js/main_modernizr_3.js"></script>
<script type="text/javascript" src="/js/main_main_4.js"></script>
````
... however, it was unable to serve them which resulted in **error 404**. This PR fixes this and allows for serving static files based on `$asset->getTargetPath()`.

Please review and send in feedback. If you like the direction I'll add tests and fix docs.