Compatibility with CakePHP 2.4

The way logging engines are loaded has changed slightly with CakePHP 2.4, as it's now assumed that the class name will end with 'Log'.  This pull request makes the Monolog plugin compatible with these changes.
http://book.cakephp.org/2.0/en/appendices/2-4-migration-guide.html#logging

The only slight problem is that after this pull request any users that update without updating their version of Cake will have to amend their logging config to change the engine name.  This is noted in the Readme.