Support HTTP HEAD in EntryResource
Should return 200 or 404.
If 200, the response should also contain the modification date of the requested entry.
A HEAD-request can, among other things, be used to perform a quick lookup on whether a context, user, group, etc with a certain name (alias) already exists.
Restlet already support HEAD by calling GET and returning the result without response body, but this is quite expensive in the case of EntryStore - we don't want to load and serialized the whole entry if the result is not going to be used anyways. This means that HEAD should have a very lightweight implementation.