Refactor remote content fetching in php shindig

1. remove dependency of GadgetContext when fetching remote content.
2. remove RemoteContentRequest->ignoreCache, use RemoteContentRequest->options->ignoreCache instead
3. remove dependency of SecurityToken in SigningFetcher, use $request->getToken()

Now, we support multiFetch with mixed signed and normal requests:
    $request1 = new RemoteContentRequest('http://test.chabotc.com/signing.html');
    $token = BasicSecurityToken::createFromValues('owner', 'viewer', 'app', 'domain', 'appUrl', '1', 'default');
    $request1->setToken($token);
    $request1->setAuthType(RemoteContentRequest::$AUTH_SIGNED);
    $request2 = new RemoteContentRequest('http://test.chabotc.com/ok.html');
    $requests = array($request1, $request2);
    $this->basicRemoteContent->multiFetch($requests);