Remove magic methods and constructors from interface definitions

Rationale:
  * classes are instantiated by the user (unless this is a specific class that Phalcon instantiates) and we don't care what parameters the constructor accepts (the user cares about that), therefore there is no reason for us to have constructors in the interface definitions;
  * magic methods are not called by Phalcon, therefore we should not require that they be present in the definition