Replace app/bootstrap wildcard with explicit entry

See https://github.com/symfony/symfony-standard/commit/e7b7b0f366465b15a29f44d7ba17287bb0b8392b
Ignore bin folder
Add leading slashes
Keep cache and logs directories
Reorder entries
Rename "Dependencies" section into "Managed by Composer"
Move bootstrap file to "Managed by Composer" section
Rename Composer section
Rename Assets section
Merge pull request #1091 from nicwortel/update-symfony2

Update .gitignore for Symfony2