Fix unused webroot

The webRoot parameters was not used during css path resolution. This assumed that the cache was built via front controller and the current directory is the webroot, which is not the case when the cache is built from cli. 
This causes the following error:
```
PHP Warning:  file_get_contents(style/email.css): failed to open stream: No such file or directory in /var/www/vendor/robertotru/to-inline-style-email-bundle/RobertoTru/ToInlineStyleEmailBundle/Twig/InlineCssNode.php on line 26
```
and the styles to not be included. This PR fixes this situation and also cleans up incorrect phpdocs and unused includes.