Adds XML namespaces support
I forgot to serialize and unserialize the xmlPrefix value for virtual and static properties
Refactorization of XmlPrefix annotation to XmlAttribute and XmlElement
Test for an attribute with a namespace not registered at root element
More tests for xml namespaces related metadata
Test XmlNamespace annotations inheritance
Modifies the namespace prefix autogeneration so it can be predicted
Improves the XmlNamespace annotation inheritance tests
Test for looking up the namespace in relation of current node.

Problem exposed by @marcospassos in https://github.com/schmittjoh/serializer/pull/58#discussion_r6226688
Fix namespace prefix in child nodes
Merge branch 'xml-namespaces' of git://github.com/ajgarlag/serializer