Fixed database layer classes to updrade cakedc/migraitons plugin for cakephp 3.0

Hi, I've upgraded cakedc/migrations plugin for cakephp 3.0 requirements.
https://github.com/topaz2/migrations/tree/feature/cakephp-3.0

Since cakephp 3.0 still heavily depends on DataSource instance which has been removed, there was some fix needed to core. However, fix given to ConnectionManager::getDataSource() fixes a lot of undefined method call called from cakephp 3.0 core,  but also adds dependency on migrations pluign. I'll give a try to remove deprecated ConnectionManager::getDataSource() call from migraitons plugin later, feel free to remove dependency whenever cakephp 3.0 core is ready to.