use request context to determine locale

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony-cmf/RoutingBundle/issues/34
| License       | MIT

we should look at the request context to determine the locale. are there any unexpected side effects of this? tests and the cmf sandbox are happy with this. /cc @Crell