Make the ParseObject construct throw a more descriptive exception

In a scenario of a developer **not** utilizing `ParseClient::initialize` before calling another object such as `new ParseUser()`, the user receives a message about specifying a Parse class name or using `ParseObject::create`, throwing them in the wrong direction about their error. This commit adds one more line to the Exception asking them if they in fact remembered to use `ParseClient::initialize` before being thrown away from their obvious mistake - this should prevent [questions like these](http://stackoverflow.com/questions/26865066/error-in-create-new-user-with-php-sdk/26872530) or #55 from occurring.