Implement zend_fetch_class_by_name() and fix new_ZendObjectData_Instance()

* In Zend compat layer, implemented zend_fetch_class_by_name()
* Added relevant tests
* Fixed infinite loop in new_ZendObjectData_Instance() discovered in
  subsequent testing. You need a cursor to traverse a linked list.
EZC: Fix handling of exception types other than HPHP::Object

* Fixed ref counts
* Added tests