Added a clear() function to Model.php (2.4 branch)

1.) Changed Pull Request from master branch to 2.4 (Old Pull Request: https://github.com/cakephp/cakephp/pull/1323)

2.) Updated return value in comment
Update Model.php

fixed comment spacing
Merge pull request #1327 from pafuinlu/patch-2

Added a clear() function to Model.php (2.4 branch)