Renamed QueryExecutor to CommandExecutor

--HG--
rename : Main/Source/NMemory/Execution/QueryExecutor.cs => Main/Source/NMemory/Execution/CommandExecutor.cs
rename : Main/Source/NMemory/Modularity/IQueryExecutor.cs => Main/Source/NMemory/Modularity/ICommandExecutor.cs
Added ConstraintCollection
Moved entity insert to CommandExecutor
Moved entity delete to CommandExecutor
Moved entity update to CommandExecutor
Initial support for cascaded deletion (needs more testing)
Updated license header