Implement EZP-21044: REST OPTIONS

This pull-request implements http://jira.ez.no/browse/EZP-21044, support for REST `OPTIONS` requests.

### Status
Feature works as expected, need refactoring & more tests.
Note: the refactoring of EventListeners is a side effect of the initial implementation approach. Can't (really) hurt.

### TODO
- [ ] Refactor `OptionsRouter`
- [ ] Test the refactored `OptionsRouter`