Add NamespaceContextImpl to servicemix-core

Currently, servicemix-eip and servicemix-drools each ship with an almost identical copy of NamespaceContextImpl. It would be better to move this to servicemix-core (where the XPathExpression classes are – which might also require a NamespaceContextImpl).

In order not to break any existing xbean.xml files, we should keep the current classes in servicemix-eip and servicemix-drools (can become empty, simply inherit from the core one) to make sure that eip:namespace-context and drools:namespace-context still exist.