2.4: PSR-4, peformance improvements, Traversable support

• Switched to PSR-4 autoloader.
• Improved performance of sorting in oderBy*, thenBy*: now uses specific array functions when possible instead of just usort.
• Added support for Traversable which does not implement Iterator or IteratorAggregate in from.
• Fixed toString in case of same keys.
• Added build integration with online tool SensioLabs.