Added first quickstart possibility - minimized configuration options for a Symfony box quickstart

Here is the first and biggest part of my quickstart integration addressed in #61. 

What has changed to the normal form? - nothing

Besides you are now able to navigate to

`/quickstart/symfony`

There you have a minimized form of the most customized settings. All other settings like vhost, db access and so on are provided by the system. 

How it is technical solved? - With events. During manifest compilation and file archive generation events are fired and listeners are able to add additional content to the manifest or add puppet modules to the archive.
In this way the old code is untouched and is free for extensions. 
I moved all the Symfony related code in a `Plugins` directory. 

Stuff still missing:
* create a Symfony project with composer (currently I get an error when I do with Puppet, but none when I create the project manually)
* webserver, Symfony version, PHP version etc. parameters pre-customizable via query string.

More things are comming soon. However I just wanted to give you the opportunity to check this code and to discuss it. All other changes are far simpler and smaller and so I will push them directly into master after this PR is hopefully merged :) (or give me a short sign that I should push these changes into the master on my own).