Add .gitignore file for Symfony3

Although version 3 of the Symfony Standard Distribution has not yet been released, version 2.5 already offers the possibility to start using the new directory structure that will come with version 3. The new directory structure introduces a new 'var' folder (containing cache and log files and other files that are changed by Symfony itself) and some other minor changes.

This pull requests basically adds a copy of [`Symfony2.gitignore`](https://github.com/github/gitignore/blob/master/Symfony2.gitignore) that has been altered for this new directory structure.

The major changes compared to the old directory structure are:
- `app/cache` and `app/logs` have been moved to `var/cache` and `var/logs`, respectively.
- `app/bootstrap.php.cache` has been moved to `var/bootstrap.php.cache`.
- `app/console` has been moved to `bin/console`.
- configuration for phpunit has been moved from `app` to the root folder.