Callers can ask for a specific capability in the URL:
$ curl http://localhost:8080/a/accounts/self/capabilities/createAccount
ok
in addition to the existing method of using /capabilities/?q=createAccount.
If the user has the capability an HTTP 200 Ok is returned with "ok\n" as
the body. If the user doesn't have it, HTTP 404 Not Found is returned.
Change-Id: I67feffea2224b3a783940c5ef2797e2800fb587f