security-checker gadget

add sensiolabs security checker.

https://security.sensiolabs.org/

i added a new database column `description` in the issue table. 
i think it is a good idea to allow markdown, isn't it?

waiting for #65 because of migrations.
always the goal should be a class as simple as possible to instantiate. This means you should set default values if it make sense. we don't have a use case for this, but if someone extend this projekt he can simply instantiate this class. And the default value isn't wrong :wink: