Migrating from 2.0 to 2.4

All minor changes:

"$Model" argument in HttpSocket is now "Model $model", according to the cake standard. This was breaking in Cake 2.4.

ServiceUnavailableException doesn't exist, so it has to be defined. Added instructions to readme

$actsAs must now specify name of the Plugin as the prefix. Modified readme instructions

Once I made these changes everything worked flawlessly. Thanks for releasing this!