MicroKernel API: returning the :hash property should be optional

the :hash property represents the content hash of node tree rooted at the property's parent node.

the :hash property is by default not included in the json tree returned by the getNodes(..) method but can be enabled by explicitly specifying it in the filter parameter.

returning the :hash property should be optional since it might be a too heavy requirement on some MicroKernel implementations. an implementation might e.g. choose to include the :hash property only on certain nodes or it might choose to not support it at all.

if however a :hash property is returned it has to obey the content hash contract, i.e. identical node trees must have identical :hash values and non-identical node trees must have different :hash values.