Added "hasData" on generic seialization visitors and other minor bugfixes

• Added public hasData function to check if a data key already have been added. #625 (goetas) (based on #466 by mowcixo)
• $context is not used #622 (olvlvl)
• Fix Doctrine PHPCR ODM 2.0 compatibility #605 (wouterj)
• Fixed type-hinting #586 (jgendera)
• Fix multiple handler callbacks in YamlDriver #515 (mpajunen)
• Fixed minor typos #364 (sdaoudi)
• Default $serializeNull to false #317 (steveYeah)
• Missing attribute 'xml-value' in XML Reference #269 (holtkamp)
• Removed unnecessary use statement #262 (dunglas)