Implement EZP-21044: REST OPTIONS

This pull-request implements http://jira.ez.no/browse/EZP-21044, support for REST `OPTIONS` requests.

### Status

A OPTIONS requests to any REST URI will receive a response.
It will include an `Allow` header with the comma separated list of methods accepted by the URI:

```
$ curl -u "admin:publish" -X OPTIONS -i http://vm.ezpublish5/api/ezp/v2/content/objects/1

HTTP/1.1 200 OK
Allow: PATCH,GET,DELETE,COPY
Status: 200 OK
```

### Dependencies

See https://github.com/ezsystems/ezpublish-community/pull/86.

### How

Uses a custom route loader to go through REST routes, and based on those, generate a new set of routes for OPTIONS call to all declared path.

The list of supported methods is passed on via a route default, _methods.

### Testing

Unit test with 100% coverage + manual HTTP requests.

### TODO
- [x] Refactor `OptionsRouter`
- [x] Test the refactored `OptionsRouter`
- [x] Update REST specs
- [x] Add `Content-Length: 0` to the Response
- [x] Create ezpublish-community pull request for `ezpublish/config/routing.yml`
- [x] Investigate possibilities of removal of the `routing.yml`