Lazy loading

Closes #89 . Changes:

* separate phpunit test file for mysql (mysql behaves differently from sqlite on GROUP BY), run with ```./phpunit.phar -c phpunit.mysql.xml``` changing config values accordingly. All test are marked with ```@group mysql```
* introduces AuditedCollection that adds lazy-loading capabilities on two levels - entities are resolved on first pass and loaded only when requested directly (or some method forces to load all the collection, e.g. ```partition()```);
* now you can audited behavior with ```loadAuditedColelctions, loadNativeCollections, loadAuditedEntities, loadNativeEntities``` calls, controlling loading of ToMany and ToOne relations accordingly. Please call ```clearEntityCache``` after you change this.