Implemented EZP-20238 - Upgrade to Symfony 2.2

**Work in progress**

## TODO

- [ ] **Semantic configuration**: Use `keepKeys()` in config tree to avoid key normalization whenever needed (see https://github.com/symfony/symfony/pull/6367)
- [x] **HttpCache**: `LocationAwareStore` should now be based on `Symfony\Component\HttpKernel\HttpCache\Store` (see https://github.com/symfony/symfony/pull/6016)
- [x] **Twig**: Default Twig loader is now `Twig_Loader_Chain`. We can now remove the twig loader filesystem override, and keep `twig.loader.string` (See https://github.com/symfony/symfony/commit/f48b22a44ea171404471f4371d5f0242705c5f80)
- [ ] Replace deprecated Symfony API calls by new ones (see https://github.com/symfony/symfony/blob/master/UPGRADE-3.0.md)
- [ ] Fix potential regressions !