[2.0] Refactored classloader architecture. Renamed ClassLoader => GlobalClassLoader. Introduced IsolatedClassLoader that is suitable for participating in autoload stacks. Added 2 example entity classes to the sandbox with 2 xml mappings and 2 yaml mappings. Simplified sandbox setup.

[2.0] Changing sandbox to use ArrayCache by default so it doesn't require APC. Added comment with APC as the recommended for production

[2.0] Updated description for pear packages. Enhanced ClassLoader to support the old namespace separator (underscore) for Zend/PEAR/... compatibility until some day when they all use 5.3+.