Over-ride Mage_Core_Model_Config to allow mocking functionality

This patch allows you to set mock objects for Mage::getModel() via the Config object,
so the designated model name will return your mock rather than what Magento would
normally return.

Example:

$myMock = $this->getMock('Mage_Catalog_Model_Product');
Mage::getConfig()->setModelInstanceMock('catalog/product', $myMock);

All future Mage::getModel('catalog/product') calls will return the given mock object.

If multiple mock objects are set in sequence Magento will return each mock object in order
until the last one, which will be returned repeatedly from then on.