Moved pre_save after validation and determination of creation state; added pre_save_validation where pre_save had been.
Added missing reference in __all__.
Added tests for pre_save_validation and updated tests for pre_save to encompass created flag.
Added pre_save_validation to signal list in documentation.
Merge pull request #345 from amcgregor/master

Addition of pre_save_validation and move of pre_save to after validation.