Added checking symfony requirements before showing success message

I've added checking symfony requirements (as per #9)
If any of requirements is not met installer will show error message and list missing requirements. Hovewer, it will not show recommendations as app/check.php does that already and I think it's not needed here?
Probably error message can be better but I think it's a good start.

Not sure really.. @fabpot @javiereguiluz ? Maybe only link to docs?
I think more imporant would be to add info about `app/check.php` so user would know how to test requirements later?