Added a new security:check command

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/issues/11146
| License       | MIT
| Doc PR        | -

**Command output when there is no security error:**

![security_check_no_errors](https://cloud.githubusercontent.com/assets/73419/3551776/6a40c938-08ec-11e4-860e-b1e14cb5fc1a.png)

**Command output when there are some security errors:**

![security_check_errors](https://cloud.githubusercontent.com/assets/73419/3551777/6d6105ba-08ec-11e4-8088-9cd2a5589dae.png)


If this PR is accepted, I'll add the corresponding tests and documentation.