Adding composer security:check

It adds a really nice feature and it fits perfectly inside composer as the base file for the check is created by composer itself

If vulnerabilities are found, it shows : 

```
$ ../composer/bin/composer security:check
Security Report
===============

The checker detected 2 package(s) that have known* vulnerabilities in
your project. We recommend you to check the related security advisories
and upgrade these dependencies.

symfony/http-foundation (v2.0.18)
---------------------------------

CVE-2012-6431: Routes behind a firewall are accessible even when not logged in
               http://symfony.com/blog/security-release-symfony-2-0-20-and-2-1-5
-released

xxx-xxxx-xxxx: Request::getClientIp() when the trust proxy mode is enabled
               http://symfony.com/blog/security-release-symfony-2-0-19-and-2-1-4


symfony/security (v2.0.18)
--------------------------

CVE-2012-6431: Routes behind a firewall are accessible even when not logged in
               http://symfony.com/blog/security-release-symfony-2-0-20-and-2-1-5
-released


* Disclaimer: This checker can only detect vulnerabilities that are referenced
              in the SensioLabs security advisories database.

```
If not : 
```
$ bin/composer security:check
Security Report
===============

No known* vulnerabilities detected.

* Disclaimer: This checker can only detect vulnerabilities that are referenced
              in the SensioLabs security advisories database.
```
What do you think ?