Zend_Db adapterNamespace #979

Zend_Db allows an 'adapterNamespace' to be specified. This would allow us to remove core/Db.php (Piwik_Db::factory) and go back to using Zend_Db::factory.

Changes:

• config/global.ini.php and config/config.ini.php.sample
• plugins/Installation/Controller.php
• core/Piwik.php

Remove:

• core/Db.php

Update script:

• add adapterNamespace to config/config.ini.php
• alternative is to set this at runtime

Let's not change this for now. The extra abstraction may help others in porting Piwik to other databases (e.g., Postgresql).