add 'atomic' option to "save()" API

This commit adds a transaction context to 'save()' API in order to rollback
possible modifications done in some 'Model.beforeSave' listener callback.
This will allow cakephp 2.x to behave like 3.0 .
It uses try/catch to better handle transaction.
Previous save() API is renamed to protected _doSave() method.
A new save() method is created for transaction handling.
'atomic' option is disabled for internal 'save()' call.
Merge pull request #3604 from Haititi/2.6-save-atomic

2.6 add 'atomic' option to "save()" API - delivery