Add ability to create Yubikey, SMS second factor

- [x] Translate translation keys
- [x] Document second factor API in [wiki](https://wiki.surfnet.nl/display/stepup/API+Design)
- [x] Decide on consistent usage of Step-up domain value objects in projection entities
  - [x] `\Surfnet\StepupMiddleware\ApiBundle\Identity\Entity\Identity::$institution` has string type hint.
- [ ] Keep a list of second factors in the agg root
- [ ] Refactor projections to have one (immutable) object per state.
- [ ] Move controller actions with identity resource root into identity controller.
I infer the following: in the future, we will store the identity's preferred locale in the aggregate. For now, when creating the event, pass in the static locale `'en_GB'`. This event property can then be used when sending the e-mail verification e-mail verification e-mail.

[Y/n]  ![image](http://img3.wikia.nocookie.net/__cb20091129041432/uncyclopedia/images/b/b8/Thin_cursor.gif)