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.

You are right about the need to display the `app/check.php` command to re-check requirements.

I propose to display the following message when **everything went right**:

![installation_is_right](https://cloud.githubusercontent.com/assets/73419/5127948/ea0ec464-70d7-11e4-83b2-8bbda99e33a6.png)

And the following message when **something went wrong**:

![installation_is_wrong](https://cloud.githubusercontent.com/assets/73419/5127949/ec04df1a-70d7-11e4-95a0-665b29960679.png)

If you agree, this is the code I used to display those messages:

```php
    private function displayInstallationResult()
    {
        if (empty($this->requirementsErrors)) {
            $this->output->writeln(sprintf(
                " <info>%s</info>  Symfony was <info>successfully installed</info>. Now you can:\n",
                defined('PHP_WINDOWS_VERSION_BUILD') ? 'OK' : '✔'
            ));
        } else {
            $this->output->writeln(sprintf(
                " <comment>%s</comment>  Symfony was <info>successfully installed</info> but your system doesn't meet its\n".
                "     technical requirements! Fix the following issues before executing\n".
                "     your Symfony application:\n",
                defined('PHP_WINDOWS_VERSION_BUILD') ? 'FAILED' : '✕'
            ));

            foreach ($this->requirementsErrors as $helpText) {
                $this->output->writeln(' * '.$helpText);
            }

            $this->output->writeln(sprintf(
                " After fixing these issues, re-check Symfony requirements executing this command:\n\n".
                "   <comment>php %s/app/check.php</comment>\n\n".
                " Then, you can:\n",
                $this->projectName
            ));
        }

        $this->output->writeln(sprintf(
            "    * Configure your application in <comment>%s/app/config/parameters.yml</comment> file.\n\n".
            "    * Run your application:\n".
            "        1. Execute the <comment>php %s/app/console server:run</comment> command.\n".
            "        2. Browse to the <comment>http://localhost:8000</comment> URL.\n\n".
            "    * Read the documentation at <comment>http://symfony.com/doc</comment>\n",
            $this->projectName, $this->projectName
        ));

        return $this;
    }
```