Adds simple support for HTTP OPTIONS requests

Lets the default action mapper know about the HTTP OPTIONS method, which
currently simply responds with an Allow header saying both GET and POST
is supported for all requests. A future more correct implementation
might introspect the mapped action class for which methods it actually
responds too.

This lets Kolibri handle requests from Microsoft Service Discovery
and similar robots which sends OPTIONS requests to map out which HTTP
methods are supported on URLs.