Allow for Monolog to already be autoloaded

Adds support for the recommended way of using Composer with CakePHP
Class name now compatible with CakePHP 2.4
Filename now compatible with CakePHP 2.4
Updated documentation now that class name has changed
Updated tests with new class name
Explain to use MonologLog for older version of Cake
Merge pull request #8 from burriko/cakephp2.4

Make it CakePHP 2.4 compatible

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.