[BUGFIX beta] Make flag LOG_TRANSITIONS_INTERNAL work again

The `router.log` method is set to `Ember.Logger.debug` when the
LOG_TRANSITIONS_INTERNAL flag on the application is set. Since the
`namespace` at the time of the check is `undefined`, this doesn't work.

This fix moves the check into the routers' `init` method.

This closes #3813
Merge pull request #3814 from pangratz/fix/router_LOG_TRANSITIONS_INTERNAL

[BUGFIX beta] Make flag LOG_TRANSITIONS_INTERNAL work again