Configuration system refactor - step 1

Configuration system refactor, according to issue 285:
- Add support for various configuration formats
- Create a more rational configuration hierarchy
- Validate the configuration values

Step 1: add new classes and relative tests
Configuration system refactor - step 2

Implement Propel\Common\Config\PropelConfiguration class to validate configuration
and load defaults, via Symfony\Component\Config\Definition\Builder\TreeBuilder class.
Remove all not more used options and define a better property structure.