Allow to query for null values/missing properties

[mwahle]

We need to be able to query for instances that do not have a particular property, as in "constraints": { "tag": [ "==", null ] } or "constraints": { "tag": [ "==", [ "valueA", "valueB", null ] ] } Of course, one could add a default value to the "tag" property but that has a multitude of implications for the application, from properly handling this special string value to making sure that at no point an instance could be inserted without that special value to not brake the entire system.

Please also see #44.

[ifadams]

@cwlacewe, reasonable enhancement but has dependency on #44