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.

The resulting Exception now displays as such:

>[11-Nov-2014 20:19:34 Europe/Berlin] PHP Fatal error:  Uncaught exception 'Exception' with message 'You must specify a Parse class name or register the appropriate subclass when creating a new Object.  Use ParseObject::create to create a subclass object. Perhaps you forgot to use ParseClient::initialize before creating your Object?' in /Users/schuylerjager/Desktop/GitRepos/parse-test/parse-php-sdk/src/Parse/ParseObject.php:88
>Stack trace:
>0 /Users/schuylerjager/Desktop/GitRepos/parse-test/index.php(17): Parse\ParseObject->__construct()
>1 {main}
  thrown in /Users/schuylerjager/Desktop/GitRepos/parse-test/parse-php-sdk/src/Parse/ParseObject.php on line 88